For a 180, I would use at least a 30 gallon tank for sump... 10 gallons won't get you far.
As for the canister filter debate... the Cascade is just as good as the FX5 and the Eheim for a portion of the cost. Eheim is going to be about the most expensive canister you'll find, and the only one that never goes on sale. The manufacturers know they have the best, so they never give the retailers any kind of sale or break on price. The extra features on the Pro III are nice, but are they really worth that much more money?
The cheapest price I could find online for the Eheim Pro III is $379.99, but I found the Cascade 1500 on sale for $139.99 here: http://www.worldpetstore.com/ca15cafi.html
If you're wanting to "over filter", its still cheaper to get 2 of the Cascade 1500's... you're still only spending $279.98 and getting plenty of filtration!
I found the Fluval XF5 on sale for $239.95 online, and after the company reps came in to train us on the XF5, me and my coworkers all found the Cascade to be "less complicated" and to work just as well. The Cascade actually has just as much space as the FX5 for media, and is A LOT less complicated to use. When the company reps have a hard time working with a filter during a training seminar... that alone says something!!! I was disappointed in the FX5 overall, but would still suggest it over the Eheim based on price differences and ease of use/maintenance.
Something else to take into account before selecting your filter is about how easy it will be to get replacement parts and/or tech support if you need it? For Fluval... good luck in finding the parts, and also in dealing with the company for them. Penn Plax seems to have Fluval beat in that area.