No canister can come anywhere close to a sump when it comes to aerating the water. Both sumps and canisters will give plenty of bio filtration. Depending on design and configuration both will have mechanical filtration capabilities. But a sump configured as a wet/dry with bioballs will supply oxygen to the nitrifying bacteria where it is consumed by the bacteria and then add plenty more O2 for the fish inhabitants. Canisters will not add any O2 and the bacteria will consume O2 from the water competing with the fish. This is the major advantage to a sump.
When well designed sumps can also be very easy to maintain. Easy to clean = cleaning more often. If you remove the waste before it is broken down to nitrates you will have much less nitrates to deal with... and have to flush them from the tank with water changes much less often. So when you design your sump (you are a real fish keeper with a 125g now) make sure the mechanical filtration is easy to get to, remove, clean and replace. Either filter pads or filters socks should give you sufficient mechanical filtration. Socks can be washed in a washing machine, filter pads are usually disposed of and replaced.
All in all it takes me about 5 minutes to replace the filter pad in my commercial built wet/dry sump. When I build my DIY sump I am going to try filter socks. It takes me atleast 20 minutes to disconnect my Eheim Pro III. lug it to the kitchen sink, dismantle it, replace the fine mechanical media, rinse the coarse mechanical media, refill with water so I don't have to reprime, lug it back to the tank and reconnect. For my sump I don't even have to turn off the filter to replace the filter media.... which would you do more often?