New Filter

I'm trying to decide which filter on the market to buy to replace my UGF plates.

I have a 40 gal tank with 20 or so fish.  I'm leaning towards a floor or
HOT Magnum Canister.  The mail order catalogue I received had a number of
filters for very much cheaper than my fish shop prices, but I am not sure
what size I should be looking at.

Is there any rough guide as to how many gals or litres of water per hour a
filter should be able to handle?

My choice is from:

(Hang on Filters)
Aquaclear Filter
Whisper Power Filter
Penguin Hangon Filter with BioWheel

Fluval Canister Filter
Sacem Marathon
Eheim Power Filter
Magnam Power Filter
HOT Magnam Power Filter

Does anyone have any comments on any of these?