Either / or but I tend to add the international features to the US carrier. I use AT&T and other carriers have other options. One of the better options is the 50 texts for $10. Phone calls in general were expensive. The data plans were helpful for Google Maps and email.

I must admit the sim card in Europe idea confuses me as one must relay a new phone number. The AT&T service worked well enough in Rome. Nothing works very well in Rome so do not be surprised if you lack service in certain areas or at certain times of the day.