I too have the issue where I have to mix with headphones. It can be done and there are many (even in this forum) that use them. Just be sure to try your mixes on as many different systems as possible to get an idea of where your headphones fall down. You'll find your ears will eventually be able to hear what's missing, or needs compensating for. I'm not a big fan of mixing on stereo systems, they are usually EQ'd in a way that doesn't represent the frequencies truly. And if your room is not treated, that adds even more issues to get in the way.
I've not mixed on that model of AT headphones so I can't help there. Maybe others can chime in and let you know what to be careful of. The fact that your getting wildly different results from the two choices should tell you that something's not right. My guess (and it's only that) is your speakers are not representing your mix correctly. I also must agree that listening to imported mixes is a great way to get started and as a continued reference.
Some good advice above. It's all a matter of trial and error when you're beginning, keep at it and your mixes will improve the more you do it.

The Grim
and arguably one of the best sonarworks reference 3 is excellent, a bit more pricey at 99 euro, but well worth it
 http://www.sonarworks.com/headphones
 

+1 for sonarworks!
 
I use this on my $27.00 Superlux 681s. 
 
https://www.amazon.com/Superlux-681-Dynamic-Semi-Open-Headphones/dp/B002GHIPYI/ref=sr_1_1?ie=UTF8&qid=1492774285&sr=8-1&keywords=Superlux+681
 
Without the curve they are very boomy.  But the curve smooths them right out!  Love listening to my reference tracks (Steely Dan, AJA and Anything Mastered by Bob Ludwig).
 
This is the Mix I did for the Mixing Practice found here: [link=http://forum.cakewalk.com/Mixing-Practice-m3537464.aspx]http://forum.cakewalk.com...Practice-m3537464.aspx[/link] The individual; tracks were available and recorded well.  I just had to put together a mix using only Sonar Platinum Plug-ins. I did use the CA-2A plugin since that was free for a while....   Let me know what you think https://soundcloud.com/patm300e/im-alright

Yeps. Sometimes headphones are all we got, sadly.
My advice for you with eq is to solo each track and just use the prochannel. Turn on the high pass filter and sweep as far up as you can before you hear the sound going into a telephone effect. In other words - you hear NO CHANGE. Then try the same with the low pass going down.  Then try and boost each instrument, voice etc in its own dominant frequency with a tight Q setting so that it has its own little place. You'll need to tweak this up and down the frequency range. Don't boost anything by more than 5. Keep all reverb to an absolute minimum and maybe just on one track even. Compression is tricky on headphones. Maybe just use one on the master rather than per track and use it as little as possible. Most presets will do what they say on the tin.
 
Good luck

As far as headphone mixing goes, I use Waves Virtual Room (for the panning) and Sonarworks (for flattening my headphones for an honest mix).  That combination gives me the best path to ensure that what I’m hearing is what’s being sent from the DAW.
After getting that, it’s a matter of learning how to mix and master.  Those are tricky tasks, and after decades of trying I’m still not as good as I want to be. I still hire a producer/engineer to mix/master the projects that I use for anything more than my own listening pleasure.

Hey Guys,
Thanks for all the advice, it's really kind of you to all chip in.
Just to update you on progress - having been alarmed by the initial sound going from headphones to hi-fi, I've managed to put together a mix that sounds pretty decent on both.
As suggested I'm trying to keep the volume quite low on headphones and next I'll try listening via laptop speakers and maybe burn a CD and listen in the car.
There may be a couple of elements to add to the song and then it's on to mastering.
I won't be sending it away for mastering because although I'd like to do well in the competition, more important is the learning process, so I will have a bash at mastering it myself.
Hopefully, I'll be able to release something onto SoundCloud in the near future and I'll post a link so you can check out the fruits of your advice!
The nature of the competition was that I was given a dozen long samples and had to incorporate them into a composition.  I decided to challenge myself a bit more and have made the entire track using only the samples, including percussion.  There was no percussion (kicks, snares, hats) anywhere in the samples, so I've used micro-samples to create, distort, twist into things more useful.  It's kind of an upbeat ambient thing!

Not sure if this has been mentioned above, but be very conscious of any post processing when comparing systems (ensure any "FX" are off). Also be aware of the speaker capability... I have old LS-15s I rarely use and the bass from them is overwhelming compared to what I expect (from commercial tracks). With headphones it is nigh impossible to gauge low frequencies accurately, which is where visual representation can help.
 
I just wanted to throw that out, since when you say "Hi-Fi" you could very likely have virtual surround, other effects, etc. going on as well.

The Grim
there is also a little thing which i think should be in sonar, made by craig anderton which can be used for this type of thing, i have tried it, and it's ok, not up to the others mentioned, but maybe worth a try, can't recall what it's called though

It's called the "Monitorizer" and was part of the Kingston update. You should be able to find it in the Anderton Collection folder. Please note it's not designed to model a room or specific speakers, but simply sound more like listening over speakers than headphones, particularly in terms of imaging. As the documentation in the eZine says...
 
The Monitorizer FX Chain, part of the Anderton Collection, is a send effect that helps eliminate the "super-wide" stereo sound of headphones to approximate an experience that's more like listening on monitors. Furthermore, some people simply like the stereo imaging better with the Monitorizer engaged when listening on headphones, as it can also introduce subtle room damping and room reflections, and you can adjust the effect amount.
 
While not as sophisticated as some products that try to emulate a complete room environment, the Monitorizer is based on the same principles:
 

• Feed a little left channel into the right channel, and feed a little right channel into the left channel.
• Delay the additional feeds by 1-2 ms to emulate that the right speaker signal hitting your left ear is delayed slightly compared to when it hits your right ear, and the left speaker signal hitting your right ear is delayed slightly compared to when it hits your left ear.
• Add in some very subtle reflections to be more like a "room."
• Reduce the highs of the additional feeds just a tiny bit because your head is in the way and attenuates the highs.

FWIW I've often been forced to mix over headphones. How well the mix translates depends entirely on the headphones. Because I use KRK monitors, and the KRK KNS-8400 headphones are voiced like the monitors, I get good results with them due to my familiarity with the monitors.
 
I even had good results with earbuds using the (believe it or not) Monster Turbine Pro Copper models. Unfortunately they're not available anymore. 
 
Here are some links you might find helpful.
 
Can You Really Mix on Headphones?
Headphones for Mixing - Subjective Impressions
Mixing on Headphones
Mixing on Headphones (has the same title as the previous but these two are different articles from Sound on Sound)
 
 
 

SGodfrey
 

1. I saw a tutorial on mixing with pink noise.  You solo each track in turn against a constant pink noise and reduce the volume until the track has just disappeared.  Finally turn off the pink noise and enable all tracks and the mix is virtually done.  Anyone tried this?  Also comes back to question (1) – should I try this on headphones or hi-fi?

Interesting article on this here:
http://www.soundonsound.c...g-pink-noise-reference

Interesting read on using pink noise as a reference. That article includes a link to download free test tones, including pink noise. But does Platinum already come with instruments that can generate pink noise?