Both cards are no frills cards without any real-time save functions or any other fancy stuff.
Rom compatibility is basically the same. The AKAIO team will probably release an update earlier than the EDGE team if there is a new game released with anti-piracy measures, but the difference is probably only going to be a day or two. AKAIO also has some wicked menu scrolling speed and is just really user friendly, but it doesn't play mp3s & movies like the EDGE firmware does (without MoonShell). All in all, the AK2i is probably a better card for a cheaper price....
HOWEVER (and this is a big HOWEVER), the build quality of the acekard is absolutely horrid. It's almost guaranteed that you're going to eventually run into some contact issues with it (especially if you have a DSi XL). My ak2i comes from a good batch of cards but it still runs into contact issues sometimes if I put the DS to sleep and walk around with it in my bag (with a case on). By the time I open it to resume my game, I usually get a black screen or freeze and I have to reinsert the card to get the dsi to recognize it. This still occurs even after I apply the sliding & paper tricks. On the otherhand... the iEDGE is probably the dsi flashcart with the best build quality at the moment (it even has teeth!).
So basically...
AKAIO > iEDGE OS
iEDGE reliability > Ak2i reliability
Man, if they fixed the stupid contact issues with the ak2i, I honestly wouldn't mind paying a good 40$ for one. AKAIO is THAT good.