what kind of tank filter should i get???

Go with AquaClear50.

The ones I like are the acuaclear's and the bio wheel's and on my 20 gal Long i have an AC 50 and a bio wheel 150 one on each end. i like to over filter my tanks. so long as the fish are not being pushed around. i would choose a filter that is rated for one above what your tank is. example AC 30 or 50 would be just fine depending on your stocking.

i'd definately get the 20 long. Give you more options and a nice size tank.

I'd either go for a penguin 200.
or
If you don't like the penguin, then go for the aquaclear 50.

Both of them filter about 200 GPH, which would turn your tank over about 10x and that's usually what i shoot for.

Ive had aquaclear 50 and havent had any problems, i havent had any other ones that you mentioned but I wouldnt advise against the aquaclear

The Penguin 200 will not fit a 20 long. I tried.. and had to return for a Penguin 150.. heh.. I also have 2 filters on my 20 long.. used to have an aquaclear 50 also, but it broke.. wanted another one, but I was donated a Penn Plax Cascade 300.. it's pretty weak, but both filters do the job.

I would say.. get the AquaClear 50.. I just love the AquaClear's.. no BS, same designed filter for many years.. it works. Penguins are pretty good, too, though.. but I loved my AquaClear.

You don't really get any HOB or HOT filters here in the UK, are you against
using a canister filter? I use Rena's XP3 on one of my 20 gallons and a
Tetratec EX 600 on another.

The Tetratec EX 600 is like Marinelands new canister:
http://www.edirectory.co.uk/pf/880/mia/pid/4447038

I always have crystal clear, particulate free water and clean, poop-free
substrate. I've raised several batches of fry in the 20 gal with the XP3 so
no, it isn't too much flow if you position the output and some current killing
rocks/driftwood in certain places. It is waaaaaay "overfiltered" but that's no
bad thing.

The Tetratec/Marineland (identical) is cheap but really well made and has a
slower flow/higher filtration volume method, rather than just turnover
(hence why I oversized mine).

I got my Tetratec for £30 and it's got similar filtration capacity to the XP2
which retails for £115, it is actual quality for cheap, unusually!

AquaClear 50