http://gbatemp.net/ reviews all of them.
The cheapest way with the most memory and still decent compatibilty is probably a Slot-1 device. The amount of RAM is limited by the size of the SD cards you buy. There is also no need for a GBA-Slot device.
The DS-Xtreme has a finite amount of memory, although it's ease of use and compatibility make up for that. It has a usb port built in, so you can change games as often as you want. Probably holds about 20 games or more. It also fits in flush, making it much nicer to actually play. With this device there is no need to mod anything. Just plug it in, and it works.
The M3 is a GBA-Slot device so you need to flash the DS or use a PassMe card. Either way you have to at least get a PassMe card. You need 1 to flash. So, it is more expensive than a Slot-1 device overall. Plus you still need to buy memory for it. DS-Xtreme is cheaper and easier.
You have to signup to read this. But, here is a chart of all the Slot-1 cards. Things like if it needs memory or has it built in, etc.
http://gbatemp.net/i...showtopic=38183Based on what you said, I think you would go for the DSlink. Removable memory and ~$60. Look into that one. Need to flash your DS though.
Passme cards are ~$25