Cyclo is the best card. It has every feature you could want but it costs a pretty penny.
Cyclo can connect to the wii, which is used rarely, and allows you to read text guides without resetting your game. Supercard also allows this. Supercard can be a hassle to deal with at times, but it's not so bad.
M3 offers other great features, though. Sakura is by far the best firmware for any card. Pretty much all other features are included here: You can save your game at any point, built in cheat engine, etc...
M3 natively supports the M3 expansion pack, enabling you to run GBA games.
Cyclo supports the 3in1, also enabling GBA games.
In the end, if your wallet affords the cyclo, hit that. Otherwise, M3 is pretty awesome.