Take a few bikes out for spins and choose the one that feels the best. Don't get hung up on the frame material. If the Al bike with carbon stays feels the best, go for it.
A word of advice, though: take a longer ride than around the parking lot. You can't really FEEL a bike in a short ride like that. See if you can take each one out for at least 15 minutes and prefereably 20 [leave your wallet at the shop, if necessary], and try to bring it up to speed for a few minutes; try some hills and get out of the saddle; ride over some less-than-perfect pavement. A bike that feels great for five minutes on perfectly smooth and flat pavement may not feel great on a century on country roads.
Road bikes do handle differently than MTBs, and you WILL notice the lack of suspension. On the other hand, you'll find that a roadie zips up hills like lightning compared to your FS bike. I doubt you've ever sprinted over the crest of a hill at 50 km/h on your MTB -- it all becomes possible on a roadie.