Thanks for your respond. I'm 5'9" and very comfortable with my Sirrus 51cm and XTC 17", lots of stand over clearance and top tube comfort. In my experience, a one size smaller frame is easy to maneuver. However I felt more tight on my new Defy size M with 30.5 in stand over height. Still deciding on either GT Grade 51 or 53 will give me a better fit and handling on off road rides. The GT Grade 53 has a stand over height of 776 (30.6") which seems to high for my legs.