I'm shopping, too.
I don't have the experience that everyone else has, but I did test-ride a 7.5 FX on Saturday. (I did not stay at a Holiday Inn Express, though, so don't expect too much from this post.)
In my on-line shopping I had decided on the 7.5 because I want a compact crank. My '87 Schwinn has a 48/38/28 crank with a 14-30 six-speed freewheel and don't like it. The 7.4 is 48/36/26 with an 11-32 9-speed cassette. That top end ratio is higher than mine, which is good, but the low end is significantly lower, not to mention more abundant.
The FX I rode on Saturday had a 50/34 crank and 11-34 cassette. I almost fell in love. There were some steep hills in their humongous and deserted parking lot, but the FX is so light I didn't need the lowest gears.
My Schwinn weighs 31+ lbs, and they told me this particular FX was +/- 22 lbs. (it had some stuff strapped onto it). Also, wheel size comes into play here (FX's 700c vs. Schwinn's 26") but you can get an idea why I'm not interested in all those hill climbing gears. If I were in Denver or Knoxville I'd have a different opinion.
The FX may be where I settle although I'm keeping my options open for another week or two. You won't go wrong with either 7.5 or 7.4. It's the best bike I've ever ridden--but I hope to ride a Domane 2.0 next week!