Originally Posted by
econjon
Also is there not a security check post immigration and customs if staying in terminal for fcl A?
Since you're arriving from the US ("clean transit") you wouldn't have to go through transit security to get to FCL A, only passport control. Although security is easy at FCT (no lines), it's still a security check, which I personally would want to avoid.
EDIT: I'm basing this on the assumption that your IAH-FRA flight arrives at the Z gates (MOST US flights do) If your IAH-FRA arrives at the B gates, then you would have another security check regardless. In this case I'd go to FCT.
You'd only get a car/van transfer to your flight from the FCL if it departs from a remote stand. Looks like all FCO flights are mainline, so you wouldn't get the car transfer if you went to the FCL.