First thing i did when i had my RIO 400 was rip out the internal, the Juwel internals are pretty good but i wanted to fit externals to cope with the overstocking i would be doing. You normally find most people tend to stick with the same brand provided they've had good experiences with them, me personally i use Fluval. Never had any problems with new ones, i had an old 205 leak a little on me a few years back but i'd bought it second hand so i replaced the outer seal and it fixed the leak, so just be careful if buying a used one. Depending on what you are stocking in your tank you could use a 206 or if you want a little bit extra turnover then a 306.
Stick with a 'branded' filter (Fluval, Eheim, AquaOne, etc), yes you'll pay a little bit more but i wouldn't to walk into my living room to find the contents of my tank emptied due to a 'bargain filter'.