We've done two
ABD's in Italy and one that Included Paris (London/Paris). Either would be a good choice but I'd lean to an Italy itinerary. Our first foreign ABD was "Viva Italia" which covered Rome, Florence and Venice. We were blown away by that one. They have become three of our favorite cities in Europe.
Which ever you pick, I'd strongly suggest you arrive a day or two early. That gives you a chance to get acclimated and builds in a cushion in case of travel delays. Use the extra days before to explore some of the lesser points of interest that may not be included by ABD.