Hi Elspeth,
It's really a personal choice, whichever you like. I played the ornamented version, because I found it more varied and interesting. (Hint: after you've learned the first half of the movement, you've nearly learned the non-ornamented version of the second half.)
I believe the ornaments are Mozart's, but in any case, musicians ornamented passages on their own routinely in Mozart's day.
Have fun!
Teresa