In my opinion, even using a DS/DS Lite, you might as well get a DSi cart. Not only the fact that you may upgrade, but just because it's newer, and probably has less flaws.
Basically, I would just recommend the Acekard2i (With Akaio), just because it's common, easy to use, and works fine. The only problem is a ton of fake carts. I've ordered several from a few different websites, and all were real, so I don't really know how many are fake.
If you don't want an Acekard because everyone else has them, just go with the M3i or R4i. I don't really know the differences, but from what I've heard, the M3i is better.
Pretty much, it doesn't really matter. Once you run the ROM from the cart, it doesn't matter which cart you have (Unless it runs it shitty).
Yes that is somewhat true, but there are flashcarts that aren't in DSi form (original R4, CycloDS). Plus DSi versions of flashcarts can be a little more expensive.
The statement about the M3i being the best is argueable, everyone has their opinion (I think it used to be as good as the Acekard, but now the support has fallen a bit).
Also there's other things besides just playing games. There's cheat support, AP fixes, RTS, RTG, Slow-Motion, Soft-Reset, homebrew support, media support, etc.
EDIT:
QUOTE(Einherion @ May 15 2010, 02:30 AM) nsdscardsale is on the no no list.