Originally Posted by Lanovran

I work at a Trek dealer, and this kind of question comes up very often, so perhaps I can help. The 7.4 and 7.5 are in somewhat different categories of fitness hybrid. The 7.4 is at the upper end of what would be considered distance/commuting bikes in the series, with more comfort features and upgrades from the 7.3 and 7.2, such as the carbon fork, ergo IsoZone grips, nicer componentry, etc., while still having the triple chainring that may help with hilly rides (especially if carrying cargo) and slightly wider tires for a bit more cushion. As others have mentioned, it does also have the option of hydraulic disc brakes, if that's a potential factor.

The 7.5, with lighter and faster road wheels, E2 tapered headtube for improved handling, and the IsoZone elastomer on the seat stay for vibration dampening, along with the 2x9 gearing, is the first in more of a "fitness" category: what could be thought of as a "flat-bar road bike," designed for riders who don't want a true road bike, but are still looking for a quick and sporty ride. It is lighter and more efficient overall compared to the 7.4, but the difference there may not be enough to notice if you're doing only about 40 miles a week.

In all, either bike would suit you very well. If you have particularly hilly climbs to worry about, then the gearing on the 7.4 may be of benefit to you, especially if you're considering putting a rack on there and carrying stuff along with you (e.g. commuting, light touring, etc.). If it's all about the workout, then the 7.5 is a very nice choice as, again, it's designed to be more about sport and fitness. In any case, I'd recommend taking both for a test ride if possible, and going from there. Best of luck to ya!