returning airport to USA Milan or Rome

Having flown to the US from both of those airports on several occasions, I’d recommend Rome and just stay at an in airport hotel the night before. MXP doesn’t have quite the on site hotel availability as FCO.

Probably a toss up, and depends more the airline you are using. I would think from Rome you may have a better chance of a direct flight to the US, or at least more choice, particularly of entry city. When we use Rome, we are able to get a US bound flight, and a departure late morning, meaning no rush to get to the airport early, we spend the night in Rome having a great meal and a last walk around.

Rome-Fiumicino (FCO) for sure.
Milan-Malpensa (MXP) is far from the city. However flights from MXP may be cheaper.
If you can buy an open jaw multi city ticket with arrival to MXP and return from FCO that would be best for your itinerary. In other words start from the north (Milan) and work your way south to Rome. Then come home from Rome.

Your advice so far is fine. Our off-airport hotel for Milan had a shuttle service, but it was not free, and it was contracted out, so there was a small element of (imaginary?) uncertainty. I'm saying that a table-tent schedule on the reception desk was the extent of the contractor's presence at the hotel. Delays are possible at both airports.

However, I've experienced outsourced shuttles elsewhere, including Costa Rica. Didn't the USA invent "outsourcing", maybe another legacy of Jack Welch "breaking" Capitalism?

Depending on where in the US you are going, one of those may be a better choice than the other.

Have been through both ( Before Covid) and had no issues as far as the 'processing' situation.

Rome seems to be the consensus. Thanks for your help.