Originally Posted by PWMTrav

You can use contactless payment for the metro - we did it a couple of months back. It's possible some stations don't have it yet, but we didn't run into any, staying mostly in the core of Rome. Cavour, Vittorio Emanuele, Piramide, Cipro are the ones I specifically recall having contactless payment right on the entry turnstile. Apple watch, apple pay, and a contactless card all worked for my family. Busses might be hit or miss, we didn't ride any this past trip. For trains, the ticket kiosks take cards and contactless mostly.

With cash, you should still carry some. Smaller purchases like a coffee, yeah, use cash if you can help it. For larger shopping trips like groceries, clothes, etc, credit cards are fine. Amex is not accepted everywhere though, so bring a Visa/MC just in case. My personal guideline is to use cash for food without service, and small purchases under ~10 euro. Card for everything else.

I have T-Mobile Magenta Max and it was great. Definitely sufficient for a week long trip if my family is any indicator. The data speeds were very fast. You may want to load up WhatsApp, since some businesses use that for texting things like reservations and takeout orders, if you don't want to call them. But lately it seems like many have moved to online reservation tools, and the rest are responsive over email. Some you just need to call, though, and your T-Mobile plan will work just fine for that.