It runs games better.
It can run new games much faster than any R4.
The new pokemon will not work on any R4 and probably won't for a few months. It's worked on AceKards since the day it came out.
R4's stop updating quickly (ie: they stop fixing games, so you can't play any new ones).
AceKard 2i is the same price or cheaper (depending on what R4 you're looking at).
AceKards are made by the AceKard team, R4s are made by dozens of companies- none of which care about their users.
The R4 firmwares are all based off either the orig. firmware (which is old, ugly and outdated) or Moonshell 2 (which you can run on the AceKard anyway).
AceKards play homebrew better. This means more emulators (nes, snes, GB, GBC, msx...).
AceKards can run AKAIO, a firmware that is better than Moonshell 2 IMO, and it doesn't require massive amounts of patching.
If you buy an R4 and come here to ask for help with a game nobody will help you.