I have the Sweet Sixteen and I love it. It's been wonderful to be able to get so many of my finished tops quilted since I got it. I don't find the bobbin hard to reach. Don't have to change often since the bobbin holds a lot. Once you get the hang of it you can do it without even looking. Since I bring the threads to the top and clip I haven't missed a thread cutter. Having the machine at the back hasn't been a problem for me. No auto threading. I don't know if any sit down has this. I haven't had a problem threading the needle but I suppose you could use one of those inexpensive metal needle threaders.
Definitely a good idea to try them all at the quilt show.