Sumps
Pros:
They add extra water volume to your system which is good because you have more time to catch problems before they get out of control. They are flexible, you can run a skimmer inside of them which is cheaper than an external skimmer usually. You can create a refugium in them to grow macro algea to consume nitrate and breed pods which is a great way to feed fish in your tank for cheap and add extra CUC. It's also a good place to put heaters so they're not an eyesore inside the tank. You can dose for minerals in them. Put your fish in time out in them. Pluss a few other things that escape me ATM.
Cons:
Chance of flooding if you don't know what you're doing. They take up alot of space under the stand. Other than that there aren't a whole lot of cons to having a sump, they can be excellent ways to filter a tank.
Canisters
Pros:
Simple to set up. Not much of a chance of flooding anything with a canister.
Cons:
They don't do much to add any extra water volume to the tank. You can't set up a refugium inside of them. Can't hide equipment in them (heaters mainly). You'd have to buy an external protien skimmer.
Those are just some reasons...I'm sure other folks can add a few things the lists. A properly set up sump/refugium is the way to go by most peoples standards for reef.
