The AC110 is a much better filter then the emp400. You get much more effective mechanical filtration while equally effective biofiltration. Though the AC110 does cost more.
As much a I love the Rena XP3, IMO the XP4 is a joke. It cost $50.00 tp $70.00 more then the XP3 yet you only get about 10 gph more of actual flow along with one more basket of media. The XP4 is in the price range of the Fluval FX5 which is more then double the flow rate of the xp4, not to mention the extra volume you get. Not sure why anyone would purchase an XP4 after doing a little research.
Actual flow rates(without looking them up) are about, XP3 @ 187gph, XP4 @ 190gph, and the FX5 @ 475-550 gph.