Sit down versus stand up is a very personal decision. You may need to try out more machines before making a decision.
I quilted on my domestic machine for many years and had the problem you mention with tension in the shoulders. The solution I finally found was to place my machine on my sewing table and sew standing up! My dh made a styrofoam surround for my machine that was lightweight and easy to move, so I had a large flat surface for the quilt. This worked well for me. However, my solution would probably not be necessary for me these days if I had a suspension system for the quilt. This relieves a lot of the stress on the shoulders and back. See Leah Day and Jenoop websites for examples of how to suspend a quilt for a sit-down machine.
Now I have a Voyager 17/Hinterberg frame setup. I have to admit I ***much*** prefer standing up and moving the machine. It is just easier for me to quilt that way. Also, psychologically it is more free for me; I feel more creative and artistic that way!
Incidentally, I think both machines you mention are actually mid-arms; just one is a sit-down and one is a stand-up. It is the length under the arm of the machine that determines mid-arm versus long-arm. Midarms run 13"-17" or so; longarms start at 18" and go up to about 28". I think all sit-down machines are considered mid-arms as they are usually 16".
My personal bias is showing, but I would recommend a stand-up if you have the space for it. I just think it's easier to get more quilts done faster. Not to mention more fun! And, you don't have to hunt with your foot for that pesky foot pedal!