I haven't seen this mentioned yet so I thought I'd add it if someone else in my position finds this thread later.
I've been speaking with a Handi Quilter dealer that specializes in HQ machines and Pro Stitcher. They are saying that the Pro Stitcher cannot be moved from one machine to another. So say I want to put it on my Simply Sixteen now, then down the road I decide to upgrade the machine head, I would be unable to use the Pro Stitcher I already own on my new machine. This sounds crazy! I'm not sure if this is a way to sell folks on a machine upgrade or not, but either way I'm not comfortable with that kind of inflexibility with something that costs so much already. If that's true, that will be the end of my Pro Stitcher research. If that is not true, it will be the end of my communication with this particular dealer.