As with most anything - you're taking more chance with a used Canister than a new one, especially since the #04 series is not longer the current series of Fluval Canisters. The #05's (205,305,405) are the current generation of Fluval Canisters and in fact the biggest reason that they went from #04 to #05 was to fix design flaws that caused leaking.
The most common point of failure on a Fluval canister is the giant O-ring that seals the lid / motor onto the body of the canister. If you don't apply vaseline or some form of aqua-safe lubricant to it every time you service the canister, it can dry out and start a slow leak.
I used a HOB
before my Fluval 205 on my 26gal, and the biggest issues I have with HOBs are:
1. You need space behind the tank to support the HOB
reservoir, and my tank is close enough to the wall to just squeeze the hoses up.
2. HOBs that use Media Bags and / or Bio-wheels are generally better than the ones that use "all-in-one packets", but you need a bigger reservoir to support that media.
3. For some reason, my HOB
would get air caught under the outflow, and it would cause these "micro-bubbles" to get shot throughout the tank - basically made my tank look like it had dust floating around in it, but they were bubbles.
On a side note - if you do decide to get a canister and set it inside a bucket - you can invest in a "moisture alarm". These are battery powered and similar to smoke detectors but you set them down near anything that might leak water (washing machine, dishwasher, etc) and as soon as they detect an excessive amount of moisture, they'll go off.