Personally, I'd suggest a dedicated slot2 card for GBA games. The 3in1 is kinda limited. Too limited for me anyway.
An M3 slot2 card would be "the shiznit" because then, you would have the best of both worlds on the DS.
I have a Supercard miniSD (SCminiSD) as my slot2 card.......wish I would have bought the M3 slot2 instead......there is no slowdown in GBA games on the M3 slot2's. The SCminiSD slows down pathetically in many GBA games.
The 3in1 relies on the slot1 card to store the actual content for it, which cuts down on how many DS games you can put on the slot1 card. I don't see that as any kind of advantage. A dedicated slot2 is the BEST way to go to assure the maximum amount of games in the 2 cards on your DS.
I don't know about you, but I like to have the "mega arcade" in my pocket, not just a couple of games. That's why I have 2 completely dedicated cards in MY DS.