as for the sizing, if you are comfortable on the 17.5" you probably wont see a big difference going to the 20".
Im 5'9", 30" inseam, and wondered if my 17.5" was too small. but this was after switching my 18.5" x-caliber MTB to the 17.5" FX. big difference in length, but Im guessing Hybrids and Road bike are supposed to be compact. I went with the 17.5" over the 20" because I read on treks size chart that
"If you’re between sizes, it’s generally best to go with the smaller size. It’s easier to make a smaller frame fit a little larger than vice versa. "
the chart below from the
trek 7.4 page, shows the differences in the 17.5" and 20" . as for my size, 5'9", 30" inseam, I dont think theres a big difference in the 17.5" and the 20"
