I purchased my long arm before sit downs were available. Having said that, I personally found moving the pencil (long arm) more intuitive than moving the paper (domestic/sit down), so it worked out. Sorry. I would spend a couple days at a national show & try all the machines several times.
My first longarm did not have a working stitch regulator and I became pretty adept at moving the machine at a consistent rate. My current machine does have a regulator and I love it, but when doing really dense quilting I turn it off. How are you at no stitch regulation when FMQing on your domestic?
The website says 88", so I would assume that. You loose at several inches on each side to accomodate the sled that the machine rides on and you have to clamp the sides. I guess I would go with the narrower measurement. If you can get 91", consider the extra 3" a bonus.
A huge plus for me with the stand up is that you don't have to prepare your quilt sandwich on the table (or worse the floor). With a sit down, you do.
The one thing I didn't consider with my first longarm was the frame. Looking at the Grace SR2+, you should be aware that it doesn't have a dead bar. What this means is that as you advance the quilt, you will also need to raise the back roller to accomodate the growing completed quilt. With a shorter arm you will need to do this with every advance. You will need to get to both sides of the frame, so count on at least 2 feet of floor space on both sides of the frame. I didn't know this and my space available made it very difficult to get to one side of the frame. New one has a dead bar!
As far as doing a quilt larger than the frame. I've only done one, and I quilted the center then loaded and quilted the sides, then joined them using the QAYG technique with a narrow sashing strip.
Hopefully someone will chime in with their experience with a different technique.