i wear a size 31 in ub101 aswell. i just jumped the exact same as you. for the st-100x, i went to a size 31 and they feel similar to my ub101 before stretch so hopefully they will stretch to the same..!
i also got a pair of st-211, and those are def skinnier on the knee area but the waist seems like i got a slight bit more room maybe because it's a little lighter denim (13.5 instead of 14.5) so they are a little more forgiving and less rigid?
hope that helps!
Edited by lascaux - 5/26/13 at 5:26pm