Depends on what ya want. If you want a full game, then get Revelations. If you are into Mercenaries and killing a lot of things to get high scores, then get Mercs 3DS.
I found Mercs 3DS to be hard to find, new or used in stores (especially since you can't erase the save data). It was with luck that I found two copies a few weeks ago and brought one, though this may or not be a problem for you. If you can get them with ease, then I suggest getting Revelations. If you can't then you should be able to find it online then all the same hopefully.