T-Mobile has done a great job of catching up in the 3G game after a late start. They have been catching up now that they have HSPA 7.2mbps 4G-like speeds ahead of the competition. They are planning on taking things one step further with HSPA+ soon which has a theoretical speed of 42mbps which is crazy.
Their current USB modem, the WebConnect Rocket, supports HSPA but an upcoming revision has been leaked that will be the first T-Mobile device to support HSPA+ speeds. The other nice thing is that this new WebConnect Rocket 2.0 will support both PCs & Macs which is a nice update. It also has a MicroSD slot for up to 16GB of additional storage.
No release date as of yet but it will probably come out once the stock of the original WebConnect runs out according to TmoNews.