Sump pros: less equipment in tank, constant water level in tank, surface skimming, oxygenation of water, more water, customizable system
Sump cons: real-estate taken up in tank by overflows, higher CO2 usage, typically requires much more DIY work, usually noisier though that can be minimized
I think canisters work great for smaller tanks and for people not interested in DIY work. I have an Ehiem Pro II on my 45 gal and it is nice. Cleaning is not difficult as I just detach the canister and take it over to the sink for cleaning. I only clean it every few months.
I think sumps work well for larger tanks. It allows you to automate some things that can become cumbersome with larger tanks and gives you a place to centralize stuff. I have a 29gal sump on my 150 gal tank. Most all the equipment is in the sump or under the stand - another reason sumps work well for large tanks is there's usually lots of room under the stand. I still do manual water changes because that's when I vacuum the tank, but water top-off between changes is done in the sump. I have two returns driven by one pump that is teed to the returns. I have my CO2 reactor on one return and the UV sterilizer on the other. That way they only see half the water flow.
Sumps and canisters both have their uses. You just have to decide which is the best fit.