If you can afford the extra few £'s I'd go with external! Your main contenders will be Eheim, Tetratec and Fluval. Now, a lot of people have had problems with Fluvals in the past, but some say the latest Fluvals (the x05 range) are pretty good.
Anyway, I have a Fluval 204 on one tank which I bought second hand, and has been working like a trooper since. No problems at all. I'd say I'm not overly keen on the tubing of the fluvals, I like the clear tubing you get on most other filter brands.
I also have a Tetratec EX700. This filter is great! I run it on a 47gal. Fantastic filter that comes with everything in the box (including media). They can be had quite cheap too if you shop around. This would be what I'd recommend.
I've only ever had 1 eheim, and this was a dodgy second hand one from eBay that was mis-described as "perfect" when infact it had been repaired etc. I never used it so I can't comment on how good it was. The main thing about Eheim filters though is that they guarantee spare parts available for 10 years from what I'm told. They are also a tried and trusted brand. You can get an Eheim classic for not too much and most people that have them say they are the best. Personally, from looking at the one I had, I didn't like the way the media was stacked, but as I say, I didn't actually get to use it.
Hope that helps
