The MSO5k and MSO7k are nearly identical, I have used both.
The Benefit of the MS7k:
+ Better LA-Probes, the connector system of the MSO7k is definitely better.
+ Higher bandwidth.
+ 50 Ohm Termination.
+ Bigger Screen (However the MSO5k is acceptable and I think has less glare)
+ Front Cover included
The Probe-Detect is not that helpful, I never needed that. For the average user, go for the MSO5k. Get some 50Ohm through-terminations.
Rigol's Front Covers are good, I recommend getting one for your MSO5k.