I have the 6600 and I love that machine with a burning hot passion! I love it so much that I've had a standing purchase offer with my local dealer if a kindly-used 6600 comes through the door, for nearly a year. I want a second one as a backup! So far, no deal. Nobody wants to let go of theirs, and I can't blame them!
I test-drove the 8900 when it first came out, thinking I might upgrade from my 6600...and left it there. I liked the 8900 fine, but it wasn't improved enough over the 6600 to be worth the price tag, IMO. If I didn't already own the 6600 I might have been tempted, but since I'm having such a love affair with the 6600 I took a pass and haven't regretted it.
The 6600 is such a tank. I've managed to (carefully! slowly!) sew through 8 layers of garment leather with mine, repairing a cuff on DH's jacket - that amazed me. That machine is awesome.
Honestly I think you'll probably be happy with either machine. They're both very, very good.