internet browsing would be better on MTS
as for gmail? Sorry to say, I still think its better on the BB. Email I believe is integrated better still on the BB, more seamless. Then again it depends how much email your doing to justify this.
As for google maps, their the same on either phone. Honestly the only advantage the MTS has on the BB is the pinch and zoom being it has a touch screen (if you want to consider this an advantage).
When it comes to Instant Messenger and social networking like facebook, I still believe the BB is best for this hands down. It's just integrated in so much better and I think that running apps in the background helps.