My .02$ worth - It's about the quality of the build and how well thought out the glass tub is, like, is the roll bar sandwich plated somehow directly into the frame, is it one of the tubs that has good steel reinforcing at key points and as much as anything what are the improvements/enhancements done to either?
I agree with Broncobowsher, the nicer the build of either one will start to separate the value between the two. Both builds of equal mechanics, improvements, paint, etc. and I'd think the metal would edge out the glass one... However, there are some killer glass ones out there now and improvements in tub development has apparently come a long, long way.