On the issue of the UGF, I've run one on a 55 gal w/ powerheads on the tops of the lift-tubes for two years and as long as you vacuum when doing the regular water changes (which you should do anyways), this is one of the most stable systems I've ever experienced. The only time I had to do a tear down on a UGF was on a tank that had been overfed and topped off for three years with no vacuuming during the entire time. That one was definitely a nitrate factory, but the reality of it was, the tank had to be neglected for three years for something that bad to happen.
The real drawback for the UGF though is that it requires a gravel substrate that is generally unsuitable for live plants. If you aren't planning on going planted, are willing to vac when you do water changes, then a UGF is a great inexpensive way to go. Sometimes there's a reason why "old school" has lasted so long.
All canister filters are not the same. Most that I have do indeed bypass the media more and more as the media clogs. I believe they do this to ensure water flow in the tank which is one of the 2 main jobs a filter performs.