I'm 5'7" with a 30-31" inseam. I ride a 15" mountain bike with an extended seatpost. After having a professional bike fit I was told I was just on the line between a 15" and 17.5". After doing ALOT of test riding I bought the smaller frame. I felt like I could really toss around the 15" bike while the medium frame felt bulky. I am very surprised he didn't have you try a smaller frame. That is so bad!