i asked the same type of question and was told to get a 110 and from my expirence after i bought it they are excellent filters it keeps my water crystal clear ....so i would go fo a 110
I have 2 emp 400s on my 75 gallon tank and have never had any problems with them. They keep my water clear. I'd say if you have a heavily stocked tank or something bigger than a 90 gallon tank then go with the ac110 . Check out Big Al's they always have good prices and the shipping is pretty cheap also.
I have 2 AC70's on my 180, along with a DIY I made out of PVC. The DIY is strictly carbon I got from the pond store, and my AC70's are strictly bio. Crystal clear.