The Eheim Pro line offers some bells and whistles compared to the Classic line that make them easier to set up and maintain. The Classic line is cheaper but needs to be primed manually and is a bit more fiddly to take apart. Assuming equivalent turnover, there's no real differences between them in terms of performance. The Classic line is known to be extremely reliable, and all indications are that the Pro line is reliable too, but the Classics have been out for 30 years now, so there's more data on them than the Pros.
Fluval filters are good, but they're cheaper, and their build quality is noticeably lower. Their reliability isn't as high as Eheim filters, and getting spare parts for fixing problems like leaky gaskets can be more difficult. But with that said, as mid-range rather than high-end products, they're perfectly acceptable and well worth using. I've used several Fluval filters over the years and never regretted it. In fact my Rio 180 has both a Fluval 104 and an Eheim Classic 2217 attached to it!
Cheers, Neale