i have used and still use the ac110 and the cfs500. Stock VS stock i would say go with the AC110 since outta the box its ready to go with a good customer support. The cfs500 needs various mods and things done to make it a safe and reliable filter, but after that it is imo of only using xp3s before the best canister i have used. If you dont mind doing the mods it is def worth the cfs500, i have never seen water that polished or clean on any of my tanks until i got this badboy. my 120 had 3xp3s on it and the tank didnt have enough flow until i swapped one of the cfs500 in for a xp3. The ac110 has decent flow but i feel like its no where near the 500gph they claim or doesnt move water well for some reason. The cfs moves water well and polishes water with minimal maintenance if you do the correct mods.
so stock vs stock get a ac110
modded get the cfs500! google it and youll find out aobut them