What are the differences? and which one is better. I have mario vs Donkey GBA because i'm an ambassador, but i'm planning on getting donkey kong game boy

The controls and several things are pretty much the same, the levels in DK are shorter, but they get harder.

DK is actually pretty good, and its visuals are more clean and easier on the eye than in MvsDK, so that's a plus.
It's obviously shorter since each level is, well, one level instead of the two in MvsDK and there are no mini marios.

I actually like DK better, MvsDK is just a little bit too much (to me) with the prerendered visuals and longer levels.

If you liked MvsDK, I guess you'll like DK too, both have pretty much the same gameplay.

Donkey Kong from 94 on GB is ten-million times better (i can say this because i am a pro at DK. BACK FLIPS AND EVERYTHING ELSE!)

The biggest differences are that there's no Mini Mario's in the first one which means that the levels aren't as long. The other one is that instead of the gimmick of having the colored buttons in MvDK, the gimmick in this one is making temporary ladders and platforms.

I played Mario vs. DK first time few years ago and liked it a lot, then I got my 3DS and downloaded Donkey Kong and loved it, after that I got Mario vs. DK as an ambassador game but it just didn't feel as good as before. I'd say Donkey Kong is better game but Mario vs. DK has some things that GB game didn't offer.

I liked the GBA game, but when I noticed the lack of some of the old Donkey Kong elements and Mario's voice being in a GBA game, I couldn't take it anymore. Get the GB version, as that will last you a while.