I would go for the 8900 if possible. I've had a 7700 for 18 months now and love it, but I test-drove an 8900 at a quilt show this summer (just kicking the tires, or so I told myself) and will be talking with my dealer about upgrading in the next few months. The 8900 seemed quieter, the 1/4" piecing spot on, and I like the idea of separate stitch plates rather than the single APC plate of the 7700.
Michelle