Originally Posted by
jlin
i've heard of lbs's offering free fitting. fyi, they say the right tt length is the most important part of fit (not that the other specs aren't as well), and i agree.
Yes, TT length is the most important part of a fit. Since you didn't explain why, I'll go ahead:
Seat-tube length can be adjusted simply by raising the saddle, to compensate for longer legs relative to a torso. On the contrary, top-tube length can be lengthened / shortened, but with drawbacks. Running a shorter stem can throw the control and steering of the bike off (say going from 100mm to a 80mm). If you have a shorter torso relative to your legs, it's better to drop down a size as you will not be as stretched out as much, or feel like the bike is riding you.
Personally, I'm 5'10 and ride a 54cm because of this very reason. You can always raise the saddle height to make it fit your legs, you can't always shorten the frame to fit your torso.