I recently went through what you are considering. My choices were between the FX 7.4 and DS 8.4. The FX is a great bike! Light, quick and fun. I could have really enjoyed it. In fact, I really wanted it!
However, when I considered my current riding considerations, I picked the DS 8.4. It is nimble, fun and fast as well. It is heavier than the FX, but when I added up everything, I knew the DS was what I wanted.

I am not ruling out an FX being in the shop one day, but where I live and the type riding I do, I don't think it would get as much ride time as the DS does.

I just completed my first organized ride last week, 33 miles. The DS did fine. The only problem I had was 10 miles of nasty headwinds. As a note, I saw several FX bikes on the same course I was riding on.

I would test ride any bike you are considering and honestly evaluate the type of riding you will be doing with it.

Good luck!