I have a 6600 and the 350e. When I bought them (3+ years ago), I considered getting a combination machine, but, like crashnquilt, I like the ability to sew while the 350e is embroidering. Plus, the embroidery machine is always ready, without setting up the sewing machine to embroider.
I recently had the 6600 in the shop for a routine cleaning, and the owner (who has been repairing sewing machine for almost 30 years) told me that everyone raves about their 6600...but he gets a lot of complaints about the more expensive machines.
If I had it to do over, I would again buy the 6600 and 350e! They are both wonderful machines!