I'm still relatively new to fishkeeping but I consider my finest move to date getting an Eheim for my 20 gallon. My old filter was nothing to sneeze at (an AquaClear 30 - which I will be using on a 15 gal quarantine tank) but the eheim was dead silent until I screwed it up by having an electrical issue with my house, and even now it's pretty much inaudible. Nothing like a HOB. Plus there's zero media bypass so you know you're getting maximum mechanical and bio filtration.
It only has 3 areas with o-rings (outlet, inlet, and the motor/pump section) and trust me it's not like the new ones are going to fail any time soon. I've taken mine apart dozens of times messing with media etc and it won't leak. We use rubber o-rings on 1800 PSI oil systems where I work, I'm not afraid of this thing springing a major leak and killing my fish / floor. At worst, several years down the road a dry o-ring or one I somehow dinged doing something stupid develops a drip and I'll buy a new one for a couple bucks.
I can't comment otherwise on the efficacy of HOB vs Canister I just know that at this point I've not regretted for a second the switch. Also if you get a kind with quick disconnects which allow you to valve it out and take it to the kitchen/bathroom/whatever for cleaning, it's a breeze. Whereas I've had trouble getting prime back in an HOB. My HOB also wasn't good at restarting after a momentary power "flicker" which I get at an alarming rate in my house.
I'm going to hopefully pick up most of what I need tomorrow, so I will take a look at them. I have had my HOB not start back twice after a power failure, but that was because it was really dirty, not because it lost it's prime. lol I have read in the past about the eheim being difficult to prime unless you have the quick disconnects, when I was looking them up online. Do they not come with the quick disconnect? Is it something you have to buy extra?