R4 only claims its fame as one of the first flashcart onto the DS "pirating" scene. But that's long since passed and the R4 team has stopped developing and halted improving their hardware/software for the R4. In another sense, R4 is good but obsolete. It cannot read SDHC cards; which is pretty important since almost everyone needs more than 2 GB of storage after a while. Not to mention R4 incompatibility with a few new games with piracy protections and such. Cheat system on the R4 is also a big "no no" because it can't read all the cheat data in a cheat file.
As for SuperCard and Acekard decision. I would have to go for Acekard. It's cheap, looks better than Supercard in my opinion, has a great development team, as well as our very own AKAIO team. Generally, Acekard has a lot more support and it has great compatibility. The build quality for the new Acekard 2.1 and 2.i is great since they chip case is enclosed with superglue instead of just clips. Although some prefer clips since they can open the cart and operate on it.
On the other hand, SuperCard is a decent choice. Its GUI looks decent but it's very difficult to create one's own skin in comparison to the Acekard. Real-Time Save is included in the package as well. Thing is, SuperCard forum support fails in comparison to the Acekard.
Both are great carts; Acekard for a satisfactory purchase for its price. SuperCard for additional in game options such as Real-Time Save. In other areas, they are both very good and very similar.