My 3DS game carrier holds 22 games, so while I've almost filled it I haven't had to make this decision yet, and won't for at least a couple weeks (but when I do it will definitely be Pro Evo 2011 because I only need one soccer game and FIFA 12 is one of the best games on the system).
In your case, I'd probably drop Ridge Racer. I own the game and enjoy it but Jonny Metts is right when he criticizes it for being repetitive. Though I haven't played DoA, so I'm not qualified to talk about that.
Jeez, 22 games? There must be some DS carts mixed in there, because I can't imagine you have 22 3DS games....I love my 3DS and all, but are there that many good games? I've got 10, and I thought I had all the good ones.
I have to disagree with Mr. Metts, though. I think Ridge Racer 3D is one of the best non-Mario Kart racing games in the last 2 years. It is repetitive, but that's solely because of the length of the game. And his other big criticism, that the road never gets crowded with cars, becomes irrelevant halfway through the Basic Grand Prix. I just started the Advanced Prix, and the most difficult thing is getting past cars that are in front of you.
Oh, and alas, despite my defense of the game, Ridge Racer will have to go, simply because it was a toss up between this and DOA, and I think I'll still play DOA once in a while because of the throwdown challenges, which are fun but hard. And I agree with Ceric, I get an itch to play fighting games once in a while. Besides, in a few weeks I would have had to resign RR into my game shelf anyway, with Mario Kart coming out and all.