First of all keep the AC110. Good idea to run redundant filtration. That AC110 will work great in tandem with another filter, whether it be another AC110, sump or canister. As far as what your choice should be, it's going to boil down to personal taste more then what most effective.
Sumps are a good choice as they can be very quiet, low cost, and effective mech filters. They can also be very noisy, expensive and poor mech filters. Which you end up with is dependant on you. Are you creative, are you willing to do a fair amount of research, trial and error? If so then a DIY sump may be for you.
If you are just looking for plug and play, then maybe a canister would be a better choice and they are a much better value then buying a manufactured, sump, overflow, return pump set up.
A pair of AC110s will effectively filter a moderately stocked 125g tank. I have done this, though it did require weekly 50% W/Cs and I would clean one of the two AC110s every week.