QS8.1 features a larger display, and has dedicated "transpose" and "seq select" buttons on the front panel. (Tho both functions are available on the QS8, only that you've got to dig into the menus to get them).
It seems to have a little improved dynamic range too.
Besides this, it's all the same (even same patches!).
As for the price diff., I cannot tell, since I'm located in Chile, but it seems you can get a pretty good deal on a used QS8.
Shop around, and good luck!