Canisters have longer service intervals, let you pick whichever media you want (with few limitations), hold more media, and usually have fewer problems because the water is filtered before it get to the impeller. These typically make better biological filters and filters for large tanks.
Downsides are having hoses run down to the floor (can be a good thing), lower flow rate (sometimes), harder to do maintenence on, cause less surface aggitation (can be fixed). While you can swap out or rinse a filter catrdige in less than 45 seconds, this takes a lot longer on a canister and you have a higher risk of spilling some water around.
I like using both myself. I clean the Aquaclear once a week or so by rinsing the pads, and the Eheim canister gets cleaned about once every 3 or 4 months. In my smaller tank, I use my corner filter like a mini canister, I have a bunch of bio media in it, and I let it get all junked up and full of bacteria before rinsing it. It looks pretty nasty, but it sure helps keep the water clear
2x Sunburst Platys (2M, 2F) // 3x Neon Tetras (?) // 2x Cherry Barbs (1M, 1F) // 2x YoYo Loaches (?) // 1x Bushy Nose Pleco (M) // 1x Clown Pleco (?) //
-30# Live Rock
-Mixed Snails (Nassarius // Cerith // Astrea // Black Turbo // Mexican Turbo)
-8x Blue Leg Hermit Crabs // 1x Banded Serpent Star // 1x Cleaner Shrimp
-1x Royal Gramma // 1x Percula Clownfish
-2x Mushroom Leather // Metallic Green Star Polyps // Pulsing Xenia // Striped Xenia // Blue Mushroom
-Remora + Maxi-Jet 1200 // 2x Maxi-Jet 600 // Maxi-Jet 400 // Visi-Therm Deluxe 50 // Current USA Orbit 2x 40w PC