To play GBA homebrew, you will HAVE to get a slot-2 card (or the EZ-Flash V slot-1/slot-2 combo). The problem with the N-card and its clones (MK5, DS-Linker, among others) is that the DLDI file for them is read-only, meaning that you can't write to the filesystem from within homebrew. You're also limited to a fixed size limit, as you can't buy more cheap SD cards to expand the capacity. I've heard flashing speed is decent, but personally I'd prefer using removeable media.
I would suggest getting your money back to get a slot-2 card. Now whether you want to get a Supercard or an M3 is another store. Keep in mind that the Supercard has slower RAM than the M3, and therefore many games will slow down during gameplay and some simply won't boot at all. I'd check a few compatibility lists before making a decision.