I would guess that a 15 is too small. You probably need a 17.5 or 19 depending on how long your trunk and arms are. With the slanted down tube on many bikes, the inseam measurement or "standover" height isn't so important. The distance between saddle and stem is also very important and this length can only be adjusted by a few inches by moving the saddle forward or backward along the rails. You can also raise or lower the bars by about an inch by moving the spacers on the stem. If you have longer than normal arms (34 inch shirt sleeve for example) then go with the 19. If you have average arms (33 inch sleeve) then go with the 17.5. If you have short arms (32-inch sleeve) go with the smaller bike. You can easily raise the seat to accommodate your legs, but you really can't shorten the length by much.