FCO doesn't have the busing drama, as mentioned by others. Even if you are transferring Schengen-Schengen, you still have to clear security at FCO (my connection was to another airport in Italy and I had to do this). One VERY VERY important thing to note is that you will need your 2nd boarding pass to do this. Delta would not give me my internal Italian boarding pass (though it was AlItalia booked on delta.dumb) so... I waited in the very chaotic security line only to be turned back. There is an AlItalia desk downstairs from the security line that you have to go to to check in and get your boarding pass. Do this before investing your time in line. I'd still take FCO over CDG.