M3 uses removable storage, currently dosn't fit perfectly in the DSLite. M3 requires you to buy a card reader and the CF/SD/miniSD card, you also have to be sure it's compatible and fast enough. M3 has excellent DS/GBA and homebrew support G6 uses a fixed internal 4Gb memory (512MB) Fits perfectly in a DSLite, or DS/GBA/SP/Micro if you change the G6 case. Includes USB burner to copy files onto the G6. Has excellent DS/GBA support but lacks support for lots of homebrew.
