Well, AceKard (2 I believe) is cheaper cart, while M3 Real is more expensive but you get rumble ram expansion (or if you decide to buy perfect bundle, you get GBA expansion too)..
I prefer M3 Real, as I'm a long time M3 user, and I've always been satisfied with their products..I tried AceKard2, but it was not good enough to replace my M3 Real (at least for me)..
I'm using Sakura from the release day, and I'm very happy with the way it works and with all those multimedia options, as I watch movies a lot on my DS..
Wait and see what will other members suggest, and then decide which to buy, you may also check the reviews for both carts to learn little mora about them (remember, reviews are outdated, but still true about lots of things)..