First off, you won't do yourself any harm by picking the wrong carrier. If your phone can register on the network, you will get the Tmobile free data and 20 cent per minute calls - there is no danger of your being charged more.

Second, if your phone gets on a network, it's probably as good as any other. It's not like you are going to discover the magic network that throttles less - the throttling is done at the Tmobile end in the US as all the data is routed through Tmobile servers.

Third, if you really want a particular network - Inquisitor on prepaidgsm.net, who always seemed very knowledgeable, said the SIM might prefer certain networks but sometimes you can get on the non-preferred network. However, to simulate the preferred network not being available, you have to try to register on the non-preferred network multiple times. On the third or fourth try it will register.