I just got done going through this decision. I got into biking 2.5 months ago with the purchase of a Trek 7100. While this is a really nice (and very comfortable bike) I realized that I was doing much more road riding than expected so I targeted the FX series while my LBS was still willing to take the 7100 back with a decent trade value.
I was looking for 3 main things when I decided to go after and FX from a 7100:
1) A bike that could eventually take me to pure road biking, which is a goal of mine, while still being approachable from a beginner standpoint.
2) A bike with trigger shifters (upper 7x00 models have these but simply aren't in the same class as FX bikes as far as road bikes).
3) Bike with different gearing (did not like large gap between 1st and 2nd on the 7100).
The main difference between the 2010 models is the frame; the 7.2 uses Treks black aluminum frame as opposed to the white aluminum found on the 7.1. The black aluminum is found on Treks mid to higher level hybrid bikes. My 7.2 is very light and nimble. If you are looking at a 2010 7.2 and the price range is around $500 to msrp make sure it has the black aluminum as there appears to be a blue earlier released 2010 7.2 with the white aluminum that goes for the same price. Another difference is the gearing and this is the main reason I went with the 7.2 vs. the 7.1. I like the spacing of the lower gears on the 7.2 as the 7.1 (and 7100) have a ten tooth gap between the first and second rear gears. This means to find the gearing in between 2-1 and 2-2 you must go to 1-2 through 1-4. No biggy but I found it annoying. The 7.2 is also geared much taller with the 2-8 gear being larger than anything on the 7.1. A third difference in the 2010's (other than general component improvements) is that the 7.1 has a seat post with a suspension. This will make the 7.1 a bit more comfy over bumps but add a tad of weight. I noticed that the 2011 no longer has this.
On the other side of the coin, the 7.1FX is still a really nice bike and I'd probably have one if it had different gearing. My future sister in law and her husband are both serious riders and love them. She used it in a sprint triathlon and performed quite well.
I had a similar thread to yours just last week:
Also, Treks explanation of the difference in the aluminums:
Last edited by knobd; 08-20-10 at 09:23 AM.