I have a trek wsd 7.6, I went and test rode a 7.4, 7.5 and 7.6 and found a pretty big difference in them. I love the carbon fork and how light my 7.6 is, and the shock absorber under the seat makes for a very comfortable ride. I hadn't bought be bike in almost 20 years either, and I figured it might be another 20 before I did again (that's how I justified the dramatic price increase- I had been looking for something in the $500 range).
I'd test ride as many as your LBS will let you, and slowly narrow down the ones that fit you and what you are looking for the best. That's the only real way to tell the difference. I also have a string of posts on a question I asked about the trek 7.5 about a month ago. As I recall some folks chimed in on the whole series.