Well, if you just want to play games, then go with TTDS. It's the cheapest, and has the fastest loading time.
M3 has lots of potential, but it has some stuff to fix.
Supercard SDHC has the looking best menu, but it doesn't have very much support from the team. It's also has lots of bugs, and some other problems. It does have RTS, and some extra features, but some said that it has some problems with certain games.
In my opinion, go with TTDS. As explained before, it's the cheapest, and the fastest. There is a problem with Contra 4, and some might not like it's menu. Other than that, it's great.