Well, Supercard sucks monkey balls with GBA games, many of them have slowdowns on SC. M3 Lite on the other hand has very good GBA games compatibility, just as G6 Lite. The difference between two is that one uses MicroSD cards and the other one has a built in NAND flash memory. Also, I would recommend buying EZIV (mini or lite version) as it has THE BEST gba compatibility and costs much less than M3. M3 is really ripping their customers of their money, the price is too high for almost the same features. Talk about brands.
Well, I already have SC miniSD (excellent homebrew and NDS but poor GBA), DS-X (excellent NDS with download play), R4 (perfect NDS, no slowdowns or whatever). And I just ordered EZIV miniSD solely for GBA. It's really so cheap and I can use it in GBA unit if I wish so.