I think that primary difference between the Trek 7000 series and the Trek FX series is in the geometry, i.e. how up-right you are versus how bent over and other factors. The 7000 series are more hybrids, in my option, between a mountain bike (or cruiser) and a road bike. The FX series moves more towards being a flat-bar road bike. As a newer rider, you won't have any intellectual feelings about geometry. You will just have to ride them and feel the difference, but generally as you get more serious about speed you would probably find yourself wanting the FX or an actual road bike. The 7000 will be comfortable but your torso will be very upright and you will catch a lot of wind. So really, you have to understand what you want the bike for and how serious you might get about cycling in order to decide. The 7000 and FX series are for very different purposes.

From there, the more expensive bikes in both the 7000 and FX series upgrade to nicer gear shifters (derailleurs), carbon forks that dampen rough roads, better tires, lighter wheels, etc., but those things might not be important to you.

And really, the FX 7.2 is identical to the 7.6 in geometry (someone correct me if I'm wrong here), so you are really getting the exact same bike no matter what level you decide on, it's just that the components are better and lighter on the 7.6. It's like getting a V8 engine in your car instead of a V4. The shifting will be smother, the bike will be lighter and faster but it's the exact same bike. You have to decide on the balance between your budget and the elegance of the bike.