You might also consider an eWin 2in1 GBA expansion cart from psxboy.com for around $14 shipped. It doesn't have the crappy rumble that the EZ 3in1 has. It also doesn't use NOR memory but rather it has twice as much of the faster volatile memory than the EZ 3in1, which means that it can load larger GBA games into the faster memory and also means that it has twice the memory available for the browser and homebrew than the EZ 3in1.
As underpressure116 mentioned, the main advantage of the EZ 3in1 is that you can load a single GBA game into the NOR memory and it will remain there when you shut off your NDS, so you can run that game repeatedly as if it were a regular GBA cartridge. I don't know how many people actually use that feature, considering it takes 1 to 2 minutes to store the game to the NOR memory and only a few seconds to load it through the faster volatile memory (larger GBA games MUST be loaded through the NOR memory because it doesn't have enough of the faster memory to load them).
Anyway...if the NOR memory is important to you, and worth the extra complexity, extra cost, and lower amount of faster volatile memory, get the EZ 3in1. Otherwise, the eWin is cheaper, simpler, and has more of the faster memory.
-Bri