Comparing say this as the gravity:
...and this as the pressure
Please Ignore- "Why else would I stick my hand into the pirhana tank...it was dirty and needed a SCRUB DOWN!" :P
The pressure filter has it pluses also such as smaller size and the ability to be placed anywhere. My problem with it also is it is under pressure therefore has a better chance of leaking.
Now with all that said, my pond is small and a pressure filter would be easier to blend in. But I'm still looking for a gravity filter. so for now all I have is a piece of foam on the intake of my pump and lots of plants.
Another type of filter is a bog filter which is a great excuse to buy more plants. This is my favorite and may be the way I finally go. My only problem is again space but I'm tired of mowing grass so less grass, less mowing and more fun playing in the mud of my pond.