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!"
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.