Is it better to have a bike fit too small or too big? I ask because I have been looking for a bike for my gf. I took her to my LBS, and had her fitted on a trek fx, and we came to the conclusion a 17.5 is best for her. So we went out looked on craigslist and found a trek 7.2 with deore xt shifters and rear derailluer, for only $150. At the time it seemed to fit well, so we bought it. After riding it though we realized it was too big for her, and when I measured it discovered it was a 20 and not a 17.5. I recently came across a specialized Vita Sport frame, that pretty much just needs tires for only $125, but is 15 inch frame. Will she feel better, and get a better fit out of this bike? It's down to these two bike because we can not find anything of the same quality in this price range. My plan would be to swap the tires and components of the two bikes then resell the Trek or maybe keep it around and try building it for myself.

Let me just add she is 5'6 with long legs and a shorter torso.