Alot of people say that the standard Juwel internal filters are rather pants. However from what I have experienced with my two Juwel tanks they seem fine. If the filter wasn't up to the job of the size tank it's fitted to, it wouldn't be fitted to it. However you need to take into consideration stocking levels and personal preference.
However I have never tried an external filter system, which are said to be much better. In-experience on my part, however had no need to use one.