Well, you're right--the big difference is the full carbon vs. aluminum with carbon fork. The Lexa series goes up to 105 (everything but the crankset and and brakes). The Madone starts at 105 (everything but the crankset and brakes), and goes from there. I believe the Lexa also comes with a triple, which may make difference, but generally you'll get the same gearing on a compact double, just with slightly different combinations of gears.

It depends on your budget, your roads, and how susceptible you are to road vibrations. My bike is somewhere between the Lexa and the Lexa S in terms of components, and it has been fine thus far, but I can certainly see that I'd want to upgrade in the next couple years. If you want to go the aluminum route, the SLX is probably a good place. The Madone might be a little more versatile in terms of upgrading the components at some point, though.

ETA: Zoom-zoom, you may want to re-read your post!
Also, the Lexa's pretty much the equivalent of my bike or your old bike--drop bars all round.