I love my 6600 & asked about the 8900. The dealer told me the 6600 was a very good machine, & it was the 7700 that was being eliminated. If it was a really good machine, you'd think they would still be making them.
If you get the 7700, let us know how you like it. For $400, I'd probably do it just for the bigger area to FMQ, provided there was some kind of warranty. If I didn't FMQ, I'd stick with my trusty 6600. Ask what the trade-in value is.
I'd also ask if my accessories for the 6600 would fit on the 7700. I know they don't fit on the 8900.