The Bernina 830LE is pretty neat machine ... suggest you read the blogs etc to get a feel for the customer satisfaction so far.
Also ... what kind of warranty is he willing to give you on the Bernina?
How old is your 6600? I seem to remember the price of the Bernina being in the low 5 figure range, so the price you quoted is roughly 50% off which is an outstanding deal. If you like the machine, there are not a lot of glaring customer complaints about the machine in general, and the warranty is intact ... I'd jump at it.
If you change your mind and really want a 6600 back ... you can pick up one up a LOT cheaper than the discount on the Bernina.