Both will keep a tank healthy, but it really boils down to your vision of what you want to accomplish and what kind of maintenance you want to do.
Here's the pros and cons just from my experience:
CANISTER PROS
Less maintenance : My current tank (soft corals, macroalgae, moray eel) has been running on a Fluval 307 for 10 months. The filter is stuffed with pot scrubbers and a bag of chemi-pure, and there's a course sponge over the intake. I cleaned it for just the 2nd time last Friday. It didn't really need cleaning. My nitrates stay steady at 10 ppm and I change 4 gallons of water a week.
Easier maintenance: I can stand at my sink and clean the filter. Takes me maybe 15 minutes to inspect and clean the contents and re-grease the o-ring; and my knees and lower back don't complain.
Startup: I've never had a canister filter not start up after a power outage or get blocked by air.
CANISTER CONS
Clutter: The intake and outtake tubes are visible, especially the bulky sponge over the intake. It is hidden behind rockwork when viewed from the front, but it's unsightly when viewed from the side. Also, the heater, cord, inkbird temperature sensor, and surface skimmer are all visible.
SUMP PROS
No clutter: you can hide most of your equipment in the sump.
Surface skimming: the weir will keep your water surface nice and clean.
Increased volume: the more water the better, plus you have an instant time-out space for critters that aren't playing nice in the display tank.
Safe dosing: if you're going to dose anything, you can do so in your sump so it's a little less concentrated when it hits the display.
Refugium: if the sump is big enough, you have a place to keep that handy macroalgae away from the herbivores in the display tank.
SUMP CONS
Pads/Socks: Changing filter media daily to every couple of days is a pain. Cleaning the socks is a pain. Some don't mind it. I would forget and find my filter socks had turned into foam machines.
Cleaning: Fighting salt creep in a sump makes you start thinking a guppy tank with a sponge filter might be the way to go.
Overflow issues: I've had snails block intakes, large air bubbles build up in overflows (causing the return to pump in more water than was going out), toilet-flushing noises, microbubbles, loss of siphon, fish ending up in filter socks, and dogs thinking the nylon return tube was a tug-of-war toy.
Access: getting on the floor and reaching into a stand to do maintenance was no problem when I was 20. Thirty-three years later I would rather do something more pleasant, like coal mining or roofing in New Mexico in the middle of August. All-in-one filters are sometimes easier to reach, but can be a pain if you don't have dainty hands. I have gorilla paws.
In a nutshell, if I'm planning on a heavily stocked tank (which I don't think I'll ever do again), or one with sensitive corals, I'm going to add a sump. If it's going to be lightly stocked with hardier critters, then it's an internal filter or canister filter for me.
