Kind of shocked nobody answered this question. The canister filter would be best placed with the input/output on the tank and not the sump. If you put the intake/outtake in the sump then you could get air bubbles trapped which could cause it to fail. Also you want it on the main tank to increase water movement. It wouldn’t help in the sump.
Edit: Also what are you using for overflow/pump to get the water to and from your sump?