They both come with 64MB on board memory, sorry no expandable memory like the Pearl. They both can be tethered. The T-Mobile uses EDGE, and the Sprint uses EVDO. Now the Sprint EVDO is faster for net surfing, but something you will probably not notice on the handheld itself, but probably only when you have it tethered as a modem.
Each has a few things that the other doesn't have. It really just depends on who provides the best coverage in your area, and what features you want that one has and the other doesn't.
Take a look at this comparison: BlackBerry