X3 Studio VS X3 Producer?

If what you have is meeting your needs, you can "live without" either version. Anything beyond basics is typically more want than need. With that said, I think Producer is worth the want. As a vocalist, melodyne will be of great benefit. Yes the eq in studio is functional and good but the version in producer is much more useful. If money is not the issue, it is a no brainer imho.

Thanks for the quick responses. What 4 virtual instruments are in Producer and not in Studio? Are the XLN Drums a must have compared to Session? And finally, I'm a pretty awful vocalist, I'm not epecting Melodyne to make me sing like a nightingale, but is it that good?

If you plan on using you're own vocals, it would be worth it for the Melodyne. Not sure about AD, as I don't use it. But quite a few people here seem to like it.

The biggest problem I had with Session Drummer was a lack of velocity layers for the nuances I like (but for most basic-semi complex drum stuff it's fine) and the fact it didn't seem to play nice with my pad based controller.
 
However those problems were quickly remedied by purchasing the Andy John's kit for Session Drummer and there are a plethora of kits available for it produced by an excellent company called Platinum Samples.
 
Addictive Drums is a REALLY excellent program though and comes with much better kits that the base version of Session Drummer does. It also has more functionality within its internal mixer than SD does but if you know how to mix drums well already then you could technically accomplish all you need to with Session Drummer and just mixing the individual tracks normally in Sonar.
 
So... Session Drummer is definitely useable if you aren't going for some really intricate/hyper realistic stuff or are willing to buy some expansion packs for it but even Session Drummer at its best with expansion packs isn't as good as the base version of Addictive Drums in my opinion.
 
You could of course always purchase Studio and see how it floats you proverbial boat then upgrade to Producer if you feel you need/want more. I'm not sure if it would cost you any extra that way though so maybe someone else could chime in.
 
Cheers.

Is Melodyne the same in both?
And would Studio give me enough to produce a small orchestra of instruments?
I feel I need to give you a bit more info, I'm also getting a laptop to run it. If I get Studio, I put more cash to the laptop and vice versa.

IMO Producer is and always has been easily worth the extra. Melodyne is the same in both and either way requires the upgrade to Editor as Essential is pretty basic. AD blows SD away.

Flashart
Is Melodyne the same in both?
And would Studio give me enough to produce a small orchestra of instruments?
I feel I need to give you a bit more info, I'm also getting a laptop to run it. If I get Studio, I put more cash to the laptop and vice versa.

Is the laptop going to have an SSD drive? If not and it is a solid state drive then I would recommend planning to put some money aside for at least a 7200RPM drive that does NOT have the word "Green" or some such nonsense in the title. You want a good, stable, FAST C drive. Many laptops come preloaded with energy efficient drives and/or lower speed drives that will have serious impact on your work. Really if the CPU is above the recommended specs and the laptop has 8GB of RAM then that is more than enough to run Sonar reliably. The problems will come in with a slow drive that is designed to fall asleep despite your Windows power management settings.
 
So yeah... no need to go too crazy on the laptop as long as it has good, recommended hardware and you make sure you do a clean Windows install (to get rid of bloatware and wonky factory configuration stuff) and do a bit of tweaking to make sure it's not gonna have things like ports, drives or the entire system falling asleep.
 
You might just save yourself significant money that way which could be better spent on a good interface (which you already have so never mind)), external drives for storage and streaming samples, a good internal drive if the included one is insufficient for audio AND getting to enjoy the benefits of Producer.
 
You would probably be able to make a much more informed and cost effective decision about this if you start a thread asking about laptops in your price range that will run Sonar. You could save literally hundreds of dollars and end up with a better system than you would have otherwise. Cheers.
 
edit: You may however already be aware of all that already. If so my apologies but as a completely cheap bastard myself it pains me seeing people spending money unnecessarily.
 
I've priced laptops that would be very capable of handling Sonar after some TLC for $500-600 and others that could give my tower a run for its money for a couple hundred more. I could probably force my three year old $260 Acer to even run X3 in a limited capacity.

I chose Studio over Producer because I saw nothing that was going to improve what I do with Sonar. We each work differently and we each will use certain tools to get the job done.
Studio is exactly the same in all regards as far as it's workings and features go. As said, Producer gives you a good pile of extra efxs and soft synths and if you want these then it's worth the extra  $150?? (depending on the deal of the week) . It's a no brainier. Look at the list. Decide if you really need them. You can always upgrade later anyhow. 
Myself I'm more than happy with what I have and don't use 1/4 of that. I have still haven't had to activate Melodyne. I prefer to sing in key. 
 
And I will add that unless your planning on doing remote recording, don't buy a laptop for a DAW. Laptops are obsolete and not a very good value for your money. 

There's actually a few parts of Plain X3 missing that are sort of critical to me.  
Audio Snap for one. 
No Pro Channel
No SPMTE / MTC support. 
No Surround mixing ( not for me but lots of people use this. ) 
Then the instruments your missing some very useful synths, a long list. 
And then a whole lot of Efxs are excluded. Most important to me was LE 64's
Studio has most of the best stuff so was way more bang for the buck than plain X3. 
I see scook already posted this link but it's required reading if your going to compare versions. 
http://www.cakewalk.com/Products/SONAR/Versions