The original R4 doesn't work on the DSi. Also the AK2.1 and CycloDS Evolution for example don't either. Just thought to mention that..

R4 clones (ie using the name R4 in their products) are cheapish and only usable if they can run WoodR4 or other alternative firmwares. (only cart I don't own from these I listed)
AK2i has AKAIO. 'nuf said.
SCDSTwo works wonders and does quite a bit of things including GBA emulation. Batter drainage is only excessive when using the CPU, not otherwise. Extra CPU adds a nice twist into homebrew, backups of commercial ROMs don't really benefit from it AFAIK.
CycloDS iEvolution, only cart with DSi mode to date, but currently the updates are a bit off and the cart doesn't work in DS mode with latest DSi/3DS firmware. DSi mode works, but isn't perfect yet. Only cart capable of running DSi enhanced games in DSi mode (some compatibility problems though).

I'd say go for the DSTwo. If you want to play GBA on a DSi/DSiXL, it's the only real choice since DSi mode GBA doesn't run with a nice framerate even though it is possible.

Also regarding fake carts, the DSTwo and iEvo don't have clones to date AFAIK. AK2i and R4 clones (name tells a lot) do have some clones and you may end up getting a clone if you buy from a fishy reseller.

I'd recommend buying from zhuzhuchina.com or lightake.com. I've ordered from lightake and received original carts of both the AK2i and R4 (in other words the AK2i works with latest AKAIO and the R4 is as original as they can be nowadays..).


-Qtis

ps. inb4 haters saying about iSmart etc, I don't own the carts and don't have any kind of experience with them. From what I've read the DSTwo does do quite a bit of stuff so that's the reason for the recommendation.