Okay, here's what I know, and I may be wrong about a couple things. I strongly advise you to do some research or your own.
I have a DSi. I have an Acekard 2i Flash Cart. The Acekard 2i lets you plug in a microSD card, which I have loaded up with DSi Roms. DSi roms are around 16-32 MB in size. (I think?)
I cannot play GBA or GB games on my DSi. Only DSi games.This is because:
1. The GBA slot is not present on DSi's. It IS on DS and DS Lite.
2. I have an
Acekard 2i.
IF I GOT THIS:
http://www.dsiplayer.us/I would be able to play GBA games (but I don't know if it lets you play downloaded DSi Roms like the Acekard 2i does.)
You see, other DS flash carts, or whatever you want to call them, CAN'T handle GBA emulators. This is because most DS flash carts don't have their own processors. The iPlayer DOES have its own processor, and therefore it gives the DSi a little boost in speed/power,
ALLOWING it to run GBA emulators and roms.
So ... DSi vs. DS/DS Lite
DSi:
+ Bigger screen! More capabilities (for newer games), better interface. Looks MUCH cooler. Cameras!
- No GBA slot, so no GBA flash carts.
(+ the iPlayer MAY be capable of running DSi roms. It probably is, but I really can't say for sure. If it DOES, this means you'd only have to buy ONE flash cart to run GBA AND DS games)
DS/DS Lite
+ DS AND GBA slots, so you can use both a GBA and a DS flash cart.
- Now you have to buy two flash carts, one for DS and one for GBA. Smaller screen. Older interface, uglier design.
(+ the iPlayer MAY be capable of running DSi roms. It probably is, but I really can't say for sure. If it DOES, this means you'd only have to buy ONE flash cart to run GBA AND DS games)
Really, it rides entirely on these factors:
1.) Do screen size and new features matter to you?
2.) Do you actually OWN any GBA games already?
3.) Will the iPlayer run DSi roms?
That's all I can do for you. I'd suggest doing some research yourself, now.