40L or 55? Running Canister in sump?

Two questions here:

1. Which is a better tank regardless of price, a 40L or a 55? They have the same surface area so if all else is equal, are those 15 gallons really beneficial to add to parameter stability, gas exchange, stocking, etc.?

2. Have come into an offer for a Fluval canister that is almost too good to pass up, but I already run a really efficient wet/dry sump. Has anyone experimented with running a canister sucking water out of the resivoir of a sump and returning it back to the sump? I do not want to run it inline with the return pump/uv sterilizer because I don't want to deal with matching gph flow.

You can run the canister in the sump, just make sure when you set it up, it's not creating a bunch of air bubbles near the intake on your return pump from the sump to the main tank.

Thanks! I think I just might take the deal. Can't hurt a tank to overfilter it. And with the canister I will have a good filter I can use if I decide to start another tank.