If you don't have a phone then I'd definitely suggest a Mifi higher than a USB stick.
The reasons that I hold mifi in higher regard are:
-USB Sticks tend to be wider than a USB port and can obstruct other ports.
-Mifi will charge via micro USB if required, but don't need to be connected permanently to function
-You don't need to install extra software on your Mac for a mifi, you connect just like you'd connect to a wireless router
-It is a wireless router, and you can have up to 5 devices connected to it.
So if you ever wanted an iPod Touch and iPad too, you oculd connect it easily.
It's a much more flexible device than a 3G USB stick.
Virgin Mobile does a mifi for $150 and an unlimited plan for $50 a month with no contract. Although they reserve the right to throttle speeds after you have downloaded 2.5GB. Actually, that's pretty reasonable.