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I'm not an expert on the sitdowns, but I know a lot of people like the Sweet Sixteen. The APQS George is another, and there are a couple more that I am not remembering at the moment. I'm pretty sure the Sweet Sixteen and George are both dedicated FMQ machines; there are no feed dogs, so no straight stitching or decorative stitching. What Bernina did you look at?
Instead of extension tables, I recommend looking at a suspension system. Here is a link to a commercial one, although some quilters have made their own out of PVC:
http://www.jennoop.com/suspenders.html