I can give you a little Felt vs Specialized info. The shop I loiter at carries these two as well, and I have ridden a bunch of them on the trainer and on short test rides. Felts tend to be a little cheaper than a comparable Specialized, but your $1800 for a carbon Roubaix is better than anything I have seen.

The Felt Z series are the more relaxed geometry versions, similar to Roubaix. On the road, the Z5 & 6 were spritely on a nice steep hill, but cranking on them on a computerized trainer they were noodley. Frame is same on both of those, but the 6 uses Microshift parts to lower the price vs. 105 ($1600 vs $2k)

The Felt F series are really snappy, racier geometry like the Tarmac. The 2011 F3, 4, & 5 all use the same frame, so if you spend bucks between them you get component upgrades. The frame is very stiff, and I got the best output wattages for a given exertion on the Felt F's and Tarmac.

Hope this helps, or at least doesn't muddy the water. I am personally internally wrestling over the F5, after riding the 4 Felts and two Specializeds. Have clearance from the War Department, but feel like going carbon is disloyal to my aluminum stable.