As others have posted, I'd go 10ft. Just imagine trying to stand a
4x8 sheet of plywood on end under an 8ft ceiling - that extra 2ft will
really make a big difference. I also like the idea of at least a foot
or so of concrete at the wall bases. It will make cleanup much less of
a chore.
We just finished up sheetrock work in the attached 3-car garage
for our new house. Due to complexities in our design, we had some
walls that were framed all the way from basically floor height and
others that sat over 1ft concrete stem walls. In other words, we had
both 9 and 10 ft walls to frame. One thing that made the job easier
was getting both 1/2" CDX sheathing and drywall in 4x10 sheets.
Pre-cut studs are also available for 10ft walls (116-5/8"). Many
people don't know about 4x10 sheets of plywood. You won't find them at
the big box, but a decent lumber yard should carry them.
Finally, be careful to maintain enough room for the garage door
hardware. I had to run some ventilation ducts and plumbing waste lines
along the middle of our garage ceiling. By the time they were
soffitted in, it put a real squeeze on the door hardware. We ended up
going with low headroom tracks on all three doors and one still had no
room left for an automatic opener (no big deal right now since this
stall is essentially shop space). And if you want a really clean look,
make sure to install wires beforehand for the garage door opener
buttons and safety sensors. I luckily remembered to do this the
afternoon prior to the crew showing up to hang rock.

Richard Johnson PE
Camano Island, WA