If you're looking to use a lot of mobile data domestically, your best and most reliable bet is going to be going either AT&T or Verizon based mobile broadband (I'd actually recommend Verizon between the two), but be prepared to pay a pretty penny, but be willing to pay for what you need. No matter how cheap T-Mobile's data is, I wouldn't recommend them for reliable mobile broadband if you're traveling a lot (especially with destinations that might fall outside of large metro areas), the data network is just too small and too much of their network is still 2G EDGE. If you already have an Android or iPhone, definitely do WiFi tethering. Make sure the device is carrier unlocked, too.
Internationally, given some of the prices quoted and the two best domestic providers proving phones potentially with global GSM band support, it might be worth looking into KnowRoaming. You can just slap the SIM sticker on over your existing SIM card and switch over for cheaper international service while abroad, no device rentals, no SIM shopping and juggling, and continued incoming calls from your domestic phone service. Works well if you want to go with an MVNO like Selectel, too.