Verizon charges $1.29/min for GSM roaming throughout most of the major countries in Europe. For T-Mobile you're looking at $0.99/min for GSM roaming in the same area. If you're doing a decent amount of chat, that $0.30/min extra on Verizon adds up. Unless you already have one of the combo CDMA/GSM (Euro only) phones.

It's always smart to do a rate comparison on company websites before travelling, as you never know when one may be cheaper than the other. In general though the T-Mobile network has a much better footprint than the Vodaphone network throughout the EU which means T-Mobile can negotiate better roaming deals, especially in areas where Vodaphone has little coverage. The Czech Republic is a perfect example as it is $0.99/min on T-Mobile and $2.49/min on Verizon.

Going outside Europe Verizon does have a price advantage in a few places (Mongolia is only $2.49/min on VZW but $4.99/min on TMO) however in major business centers such as Australia, Hong Kong and Singapore you're looking at $2.49/min on VZW and $1.49/min on TMO). If you're going to Japan, VZW doesn't even offer roaming service.

A more accurate description of Verizon is "it's usually going to cost you more, but every now and then you'll catch a break and get a deal."