in that case it really makes little difference...both frames are essentially the same with the 7300 being a bit heavier because of the fork and build components, but that's offset by the steel fork on the 7.3fx...the 7.5fx has a lighter aluminum one...the 11-32T cog sets are the same, the fx is nimbler because of the head tube angle and straight fork, and your riding position is a little lower on the fx because of the lower bars

the fx comes with those reversible clipless/platform peddles that are a POC and both bikes have uncomfortable seats

With our Treks, the weight difference is now negligable because both bikes now have carriers and panniers, and carry the tubes, tools, pumps, and stuff that we always pack on our rides

bcoppola's advice is bang on...swap the tires to 700x32c slicks and it will be difficult to notice a major performance difference between the two bikes, I did this with my old 730 multitrack and noticed a big difference...bar ends are a good idea too...i'll have to check that out on Mrs. M.'s 7300 and see if we have room with her grip shifts

here's how ours looked when they came home