The components on the 7.3 are worth ~$250 more than those on the 7.2. That basically means it will last a little longer without needing to replace anything. If you're planning on using the bike for more than 5 years, and the $100 won't hurt you financially, it's probably worth the upgrade. But it's not a clincher.
Disc brakes are not a good choice unless you need them (i.e., if you're riding through deep mud, not just rain or snow). They are more difficult to maintain than rim brakes, and rim brakes offer more than enough stopping power.