I was thinking of going with Boost, actually, but finally went with T-mo. The allure of EDGE caught up to me.

Oddly enough, there were, for a while, 3 "best plans" in the US, depending what you were after; T-Mo with unlimited data (tethering allowed) for $6 on top of a $30 voice plan, Sprint's SERO with blazing fast unlimited data, and tethering readily sneakable for $30, IIRC, (and a contract), and Boost, for next to nothing, no contract, and unlimited, but slow, data. And each of the three uses a different, incompatible technology. That, in a nutshell, is the reason a single integrated radio is bad. (Cost and size, of course, is why having three radios is bad.)

An empty slot, with modules available for various networks, is a clean solution that fixes everything. The only reason not to implement it is lack of a really suitable standard for the cards. (Hint to Nokia, start working on one!)