AU Tiger

Since the only bike I've owned as an adult is the one I have now, I can't really make a comparison. The general consensus on this forum is that carbon does make a noticeable difference, especially when compared to aluminum (as opposed to chromoly steel). Given the small difference in price in the models you referenced, I wouldn't even consider the 7.3.

The FX 3 has an 11-34 cassette, and the 7.4 has an 11-32. This gives the FX 3 a very slight advantage if you will be riding a lot of hills. The 26-34 combination produces a 0.765 gear ratio, while the 26-32 produces 0.813. In reality, you may not even be able to notice that difference, but technically the lower ratio will make it more suited for climbing. The trade-off is that since they are both 9-speeds, there will be a bigger gap somewhere in the progression for the 11-34 compared to the 11-32, which would matter (again, very slightly) if you ride more on relatively flat roads.

Other than that, it's just the Deore vs Alivio rear derailleur. Deore is one step up from Alivio. But if you're comparing this year's Alivio to last year's Deore, it may be the same. And if that's the case, then you might also expect that on the front derailleur, this year's Acera is better than last year's Acera. If so, then the FX 3 would be slightly better than the 7.4 FX. But understand, I'm just guessing about the FX 3 being newer than the 7.4 FX, and about the year-to-year comparison between Deore and Alivio.

So all-in-all, there's not much difference between those two bikes... not enough to justify the $70 price difference. If I'm right about the FX 3 replacing the 7.4 FX in the Trek lineup, then your LBS might drop the price of the 7.4 FX to match the price of the FX 3. I know my local Giant dealer did that this year with the 2016 models when the lower-priced 2017 models were released. If they do that, then it really would just boil down to a color preference in my opinion.