The Verizon and AT & T plans are too expensive to use which will cause you to ration your usage. As a result, you may miss out on Google Maps amazing directional abilities, recently improve with Augmented Reality (point your camera at a building and it points the way!).
The best option in my opinion is to get a SIM card with as much memory as you can. First, you have to make sure you have an unlocked phone BEFORE you attempt to get a SIM card. Next, check out data plans from the best carrier in the country you are going to visit. We visited Ireland for eighteen days this past month and then finished the trip with ten days in Paris. We got a Vodafone data plan for 20 Euro for 20 gigs of data usage which came with an Irish phone number. We visited a Vodafone store and the salesperson got us up and running in about ten or fifteen minutes time. Since the plans were so inexpensive, both my wife and I got SIM cards.
They worked great in Ireland where Vodafone has the best coverage and so-so to pretty good in Paris.
I don't want to get phone calls when I am in Europe. If someone needs to reach me, they can email me and, if necessary, I can call them back using Skype, Google voice, What's App, or FaceTime.
EU regulations have ended country roaming so if you buy a SIM card in one part of the EU, it will work in another EU country. Thus, my Irish Vodafone SIM card worked seamlessly in France.
Having both my wife and I armed with phones came in handy when we wanted to find one another in the Louvre or when dinner reservations were required. If you are on a Verizon or AT & T plan at $10 a day for each day you use the service, you might foolishly economize and not take advantage of technology. With a plentiful supply of data, you can freely take advantage of Google Maps, check out its new Augmented Reality feature, and be a whole lot happier easily finding your way around the City. (sure, you can load Google Maps offline for free but you miss out on the extras like restaurant locations, reviews, etc.). When it comes time to fly home, you can download your boarding pass and get flight notifications.
Also, using your own cellular plan is more secure than using a shared WiFI from a site that meekly identifies itself as "hotel WiFi" or "Free Local WiFi" that may expose you to security issues and hacks.
In summary, get your phone unlocked, and sign up for a short term SIM card data plan. Make a few practice calls from the US with Skype or one of the other services to call back to the US when you are in Europe. For Europe to Europe calls, the European telephone number to which you are assigned will enable you to make those calls directly. Give it a whirl--it's not that hard.
Good luck and happy travels!