From what I've read, the CS6000 just has more memory.
And it's built into a CS4000 case (different styling).
The CS5000 is perfectly dandy for FTA use, though, and is well liked by the members here.
Both have been discontinued by their manufacturer and you cannot buy a new one with a warranty any more.
If you get either used at a cheap price, that'd be fine.
However, I agree with the comments above that the hot deal for the next few days would be a 
Mercury II from SatAV.
Or, you might also consider the 
Visionsat IV200.  
With the addition of an external USB hard drive, it will record shows.
Both have been reviewed well and discussed extensively on the forum.
(
and I have one of each)  
A key feature of all these receivers, is that the have blind scan capability.