5000 and 5500 of course have 2-way remote as well.
As a final difference between 6500 and 7000, the latter has RDS. However, it adds nothing unless you have a Beolink 7000 - the data is not shown anywhere else.
I would forget the 5000 as the remote control is not compatible with anything else (but has the advantage that you don't necessarily need a remote!). 5500 is a suboptimal choice if you ever plan on getting Powerlink capable speakers.
So, in my books that would converge to 6500, as they usually end up clearly cheaper than 7000. And personally I like the polished alu parts more than the brushed ones on 5000/5500
Oh yes, one very small difference that can be big if you plan on using the timer functions - the clock in 5000/5500 will not hold over power breaks. 6500/7000 have a real time clock chip with battery backup.