There are three major differences between the 7.3 and the 7.5--besides the price.
1) Fork. The 7.3 has a steel fork, the 7.5 has a carbon fiber one. There are religious wars between fans of each. Steering away from that, I've owned a low-end aluminum Trek with a steel fork. After being hit by a car, I replaced it with carbon fiber. The difference was night and day. The carbon fork has a much more forgiving ride.
2) Drivetrain. The 7.3 has a low-end MTB crankset. The 7.5 has a road crankset. The 7.3's crankset does not have replacable chainrings (the gears). When you wear out a gear, you have to replace the entire crankset. When you wear out a chainring on the FSA crankset, you simply replace the one gear. How long they last is largely a matter of your conditions. I ride every workday, no matter what the weather. Upstate NY winters are murder on parts. I got about 10,000 miles out of the middle ring on my primary commuter's triple. Fair weather riders get three to four times more.
The difference bwteen the triple on the 7.3 and the compact double on the 7.5 is largely one of personal preference, local terrain, and your power output. I'm a flatlander and compact doubles drive me nuts, because at my power output, my cruising speed is right between the two rings. I'm either cross-chained in one, or cross-chained in the other. Both stronger and less strong riders love them. I'm in the middle--too strong that I run out of gears in the 34, but not strong enough to push the 50. Triples (or standard doubles) work better for me. But what works for me may not be the best choice for you or for anyone else.
The shifters are also different between the two bikes. I don't ride mountain bikes so I'm not qualified on the topic of differences between models of MTB shifters. However, in Shimano's road equipment, R series is always pretty good stuff--above average, verging on excellent.
3) Wheels and tires. This one's tough because there are so many variables. Weight and number of spokes don't necessarily mean stronger or weaker wheels. I had a bike with heavy 32-spoke wheels (like the 7.3) which were crap--lasted about a year. I also had a Trek that came with lightweight Bontrager 24-spoke wheels (like the 7.5). Those too were crap and lasted for only a year. There's also the matter of your weight, how you manage that weight on the bike, your terrain, and your riding style.
My advice? Look at the 7.4. You get the carbon fork, but for a lot less money. Then replace components with better quality when the originals wear out. Meanwhile, use the money you save over the 7.5 for quality accessories--lock, lights, fenders, rack, panniers, and so on.