For the money, the EZ-Flash 3-in-1 expansion is the most versatile in terms of adding GBA support to an R4. You can get it for 1/4 the price of a G6 lite or an M3 Lite + microSD card, and it's much less limited than most people seem to think.
You can play most games from the RAM on the 3-in-1, the same way you would with the M3/G6, but games over 16 megabytes and games that use 1024 kbit flash saves have to be flashed to NOR. There aren't that many games that big, or that use that type of save, so it isn't much of an inconvenience.
Once you flash a game to NOR it starts up instantly every time until you flash a different game. It takes up to about 4 1/2 minutes to flash the biggest games to NOR, small games take less than a minute. On the G6/M3 it takes 25-30 seconds to start a 32 meg game every single time.
The 3-in-1 can be used for GBA linkage with the R4 (for every game except pokeyman), the M3/G6 can not. You can use GBA linkage with the M3/G6 (for every game except pokeyman), but only when you run the DS game from the slot 2 cart.
On the 3-in-1, you have to patch some GBA games with the EZ4 Client to get them to save. On the G6/M3 you have to either run the game through the game manager to create the needed save files, or create the files manually. The EZ4 client and the G6 game manager can both do multiple games in a batch so that you don't need to deal with them one by one. I haven't used the M3 game manager, but I expect that it can also do multiple games in a batch.
With the 3-in-1, GBA roms and saves are stored on the slot 1 cart. The M3 and G6 use microSD cards and internal flash memory, respectively.
The 3-in-1, the M3 and the G6 can all be used as a RAM expansion for the Nintendo web browser, though I still haven't figured out why anyone would want to use a really slow browser with a really low resolution display.
The 3-in-1 has rumble, the M3 & G6 do not.
The M3 & G6 both have a real time clock so your GBA pokeymans can tell what time it is. The 3-in-1 does not.
The 3-in-1 costs
$19 USD shipped. The M3 and G6 are about 4x that much, but I don't know the best place to buy the english versions of those carts so I won't quote a price.
