It depends on what you wannabe. Do you want something that is more road oriented, i.e. lighter, quicker and a little more aggressive? Or do you want something that is heavier duty, more mellow and relaxed?

The 7.3 is more road oriented with a flatter stem and more road bike feel and geometry. It's basically a road bike with flat bars. It will slice through the air a bit better than the 7300. But it will feel more rigid and be less forgiving in the ride.

The 7300 is more mountain bike oriented with a taller stem and a more relaxed feel. If you ride some dirt paths, it will be more forgiving than the 7.3. It also has some suspension to ease the bumps of the world. But suspension also robs you of some power.

Depending on what you want to do with the bikes, I'd suggest coming up with a little more cash and jumping into the deep end of the pool, rather than spend time wading around the shallow end with hybrids. If you want to ride road rides and go fast, get a road bike. The Trek 1.2 isn't bad. If you want to dabble in mountain bikes, get a mountain bike. The 4500 is a very good mountain bike.