Tell me about cannister filters

A canister has the main advantage of having lots of room inside for media of your choice. Whenever you decide that you need more or less of a particular function than you have, just open the filter and change what's inside and you basically have a different filter. It will move about the same water as before but now it is doing a different job. Another advantage to a canister is quiet. When I decided to get back into the hobby, my wife's first reaction was that she did not want the constant water noise of a filter. The canister is set up to return the water a few inches below the water surface. It still stirs the surface but there is no splashing sound like so many HOBs have.
To me those are the big differences. The canister is almost as easy to clean as a HOB and does not need cleaning quite as often so that part ends up being about the same. In the case of a HOB, after you clean it you need to fill it before you start it back up. In the case of a canister you need to wait for it to fill itself before you start it back up.
The initial set up is just measure and cut hoses the right length to go from the tank to the filter. Otherwise its a lot like setting up a HOB. You decide where you want the filter, where the water will leave the tank and where it will return, how you want to trim the top to fit the filter or filter hoses, etc.
When it comes to cost, I like the Rena XP series and would expect that a 75 for a goldie would use the XP3. I have also used the Marineland C series and found them nice to use. I have never used a Fluval or an Eheim so I have no opinions about them. They are likely just as nice as the ones I have used. There are lots of less well known brands around but I am not that adventurous, I have stayed with the better known brands for ease of finding parts, filter media and similar issues.

There are a number of folks who will agree with me that there are 3 good brands of canister filter to use. Eheim is one, Eheim is another and then there is Eheim. They are not for the budget constrained.

On my 75, which has a very heavy stocking level, I run an Aquaclear 300 gph, a HOT Magnum (w/ Micron cart) and an Eheim Pro II 2026. The filter has been running great the past 5 years. My other 2026 has pressurized co2 pushed through it and has been running longer.

While the newer Eheims can be pretty pricey you could pick a eheim 2217 classic for around $120 its a little less user friendly than the newer models but a great filter.

If money isnt an issue Id go for a Eheim Pro 2 2128 with intergrated heater

The Rena xp3 is another one I use that you might consider its a snap to use but the price has gone up recently and it comes with no bio-media

With goldfish, I would say your suspicion is probably right! You may also consider going the sump route.

The hardest part about the sump is figuring out the flow rate and what pump you'll need if you're diy'ing your own overflow, maintaining it will be easier than both canister and hob, because it's more accessible.

If you're still looking for a canister, get on ebay, I recently picked up a fluval 404 for around $30.

I am sure the other brands of canister filters are good filters also, but I currently use two fluval 404's on a 125g, one 404 on a 75g, one 403 on a 55g and they have been working great for me. The only thing that the fluval filters have is a ribbed lines, that I usually flush out once every year and a half to clean some of the waste that tend to collect in the lines after a while. The 405 is just a newer version, which the only thing that notice clamping of the hoses on the aquarium edge, which is a little easier on the 405's.