Gph rule different for canisters?

The canister filter has to work harder to circulate the water as it has more media to push it through and as RB said, no bypass.

Cannisters are far more efficient than a typical HOB. HOB is the ticket for smaller tanks. The larger the tank is, the better filter required. Cannisters and wet/dry handle the requirements well and at a lower turnover rate.

This is not to suggest that HOB's are faulty. They have their place just not on a larger tank.

canisters are much better because they cannot have bypass and they filter through alot more media then your typical HOB filter. Eheim is a workhorse pump and after doing a good deal of research, they had the least problematic reviews. Many canisters have bad o-rings and eheim seems to have the least problems with that. The gph is usually overrated, i think some do it with and without media inside, so its usualy wise to get a next size up and try to get one with flow control incase its too much.

Many people also do a HOB filter and a canister to help further water clarity and increase circulation.

Personally, my plans are to get an eheim 2213 or 2215 canister (classic series) and using both of my HOB's one is an aquaclear to help jump start the nitro cycle (has 3 types of media) and my whisper 10 for activated charcoal (the only media in the biobag anyways) to help remove odors from the water and just do what charcoal does. Charcoal also needs to be changed more frequently then most other media so its easy access is a plus.

here is a little perspective:

7-10x per hour for HOB filters (if you can provide more, great)
5-8x per hour for canisters
3-6x per hour for wet-dry/sump

water bypassing the media

You also have to remember that cans have head pressure to deal with - it takes more force to push water up 3' or more of tubing than it does to drop it back into the tank from 1" above the water line.