Hi: Sorry if this is an idiotic question. Am new to biking. Plan on getting a Trek 7.3 FX. I am 5'7" tall & a bit longer in the torso than in the legs. Have been told by two bike shops that I'm between a 17.5" & 20" frame.

Tried a 17.5 inch in the 7.3 and a 20" in the 7.2 (they didn't have the 20" in the 7.3). Neither felt dramatically different. Both were comfortable, but the 20" may have felt a bit more comfortable.

My dilemma is that I can get a great deal a on a barely used 17.5", but will have to pay full price for the 20".

Is there that great a difference given that they both seemed pretty comfortable with the edge to the 20"? Don't want to end up being cheap & regretting the used bike purchase.

Will not be riding more than 30 or so miles a week. At least that's the plan for now. Thanks so much for any advice.