As much as I support the G6, and I do prefer the M3 to SC in many respects, I think it really boils down to the GBA support. Some might argue the difference between SC and M3 in terms of DS compatibility, but the truth is that they are very similar, too similar if you are only buying the card for DS games.
However, in terms of GBA compatibility, if you value playing many GBA games, the differences in how M3 handles DS games and SC handles DS games are completely not the point you should worry about when choosing between them. GBA compatibility and GBA save systems between the two cards are enough that the M3 completely outweighs the SC in my eyes by far, and was easily worth the extra 30 bucks. People might say that the SC does DS games better (only slightly better), faster, etc. but if you want GBA compatibility/perfection, the M3 ends up outweighing the SC by far. Just my two cents.
However, in terms of GBA compatibility, if you value playing many GBA games, the differences in how M3 handles DS games and SC handles DS games are completely not the point you should worry about when choosing between them. GBA compatibility and GBA save systems between the two cards are enough that the M3 completely outweighs the SC in my eyes by far, and was easily worth the extra 30 bucks. People might say that the SC does DS games better (only slightly better), faster, etc. but if you want GBA compatibility/perfection, the M3 ends up outweighing the SC by far. Just my two cents.