DS lite, no contest.
GBA SP was too small for my large fingers, otherwise it might have fit the bill. Also, no headphone jack?!? But the way it was, it was far easier to play on an old GBA than on an SP - though at that point I mostly just played GBA games on the Gamecube with the Game Boy Player anyway.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/