At the risk of asking the obvious, are you all using the same drum VSTi/samples? If not, that could explain quite a bit.

Also check that the MIDI ticks per beat are set the same in all the DAWs.

I sometimes collaborate with a couple of guys who use Logic and as we live a long way apart we email each other MIDI files regularly, particularly drums. Never had a problem in either Sonar or Logic with the MIDI output of either DAW. Or with Cubase come to that.

Sonar is very, very (very) unlikely to be adding in stuff in an exported file or a MIDI track that's been dragged onto the desktop that isn't in the track when it's in Sonar. If their are any glitches, odd repeated notes, whatever, they will show up in Sonar's MIDI event editor. It could be that you've created one note and accidentally placed a longer one over it (or vice versa). This can be difficult to spot in the piano roll view because the shorter note can be covered by the graphic for the longer one. To check either use the event editor or just drag one of the offending notes out of thenway and see if there's another lurking hidden beneath it.

As suggested, the easiest answer might be to simply bounce the MIDi to audio in Sonar then give them copies of the audio.

We both use the same drums, Toontrack´s EZ Drummer with the same original soundpack, does not seem to make a difference.

Are there a any sort of velocity offsets that may be in play in their Reaper setup? That will cause ghost strokes to sound like hits.

I just done a test with a drum track from a project in X3e Producer, it was for another reason but anyway, just dragged the midi track from X3 to the desktop, then imported it into Reaper, no problems whatsoever, everything is fine.

Just finished recording an album where the drums and guitars where captured and composed using Reaper, the Keyboards wrote and recorded using Sonar, the mixing is Cubase.
Some of us use Desktop Windows 7, other use Laptop with Windows 8.1 and others Macs...
Not a problem transferring files between machines, either midi or wav.
 
One thing i notice is, everyone who was not using SONAR actually got their antenas up for it, the drummer even converted :P

I signed up with Reaper years ago after hearing some of the hype how it was on par with major DAWs. I tried it several times,I couldn't make it work.
When my main music PC died about a year ago my older PC had Reaper on it and I started to try to use it, mainly as some band mates jumped to Reaper from Protools and I was intrigued.
It wasn't until I watched the series called Reaper Explained by Kenny Gioia that I realized how powerful reaper was function wise and how user definable it is. And it differs here the most to Sonar. Sonar is easier to do some advanced using if you need to. Reaper is not a default set up to work as well as Sonar and requires lots of menu to get familiar with but you can learn these and the tutorials on line help. Not as good as the Protools ones. Man no wonder people use PTs! It is about as hard as Reaper to use but all the online info mean you can learn it.
I got my new i7 PC and using the old drive got back into X2. It looks good and is familiar but I miss the mute options in reaper the most. It feels a little constrained too in that you can only do things the way Sonar wants you to do things which is mostly fine as I don't do much in either.
Another series has come available called Reaper Mainia by Kenny Gioia.
I really like his style of teaching and some of the features really open up the possibilities in any DAW.
It is nice to get back to the look and feel of Sonar and after the experience I have learned, looking at how to better use X2 and only have days left to capitalize on the lifetimes promo for platinum! C'mon, Chrissy bonuses!
 

Render your drums to audio for them.

BTW, Reaper is a very cool DAW.
 

Yes, a very cool DAW.
 
My opinion only...
 
Workflow and mindset are so different. You come from years of Sonar familiarity, open Reaper. Nothing fits, alien landscape. Allah, those renegades don't even have a defined midi vs audio track. How can you proceed without this carved in stone paradigm.
 
Nothing works like in Sonar. Case closed. Plus the $60 price tag is already suspect.
 
Each DAW has hi and lo points. When firing up keyboards, the beauty of Reaper is to drag and drop multiple vst or vsti instruments in a chain. Slam together several synths on the same channel. I know that you can do the same in a more crippled way in Sonar. Create multiple tracks, clone multiple midi files once they are right. Save the template for future use so you don't have to go through the work again.
 
From my experience, with audio input, meaning vocals or a direct guitar, there is no advantage of one over the other. Both do the job. Chaining effects is equally efficient.
 
Handling of midi synths is a major departure between the two softwares. Reaper wins hands down. Only request is to actually engage and evaluate the differential before flaming me to the brimstone pit.
 
John
 
 

I always thought Reaper's idea that a track is ANYTHING you want it to be was pretty cool.
 
I just never wrapped my head around it in a practical sense. Maybe if I really sat with the product for 100 hours or so I'd have a handle on it.