Bridging is not the same as tethering, though your experience with your carrier may varry. AT&T has been anti Bridge though I understand their stance has softened a bit of late.

When connected via the Bridge your data should come out of your BlackBerry phone data plan. When tethered it depends on the tethering agreement you have with your carrier. On my Rogers account in the end it doesn't matter because I get free tethering but it comes out of my BlackBerry phone data allotment.

My advice if your phone is a BlackBerry that can run Bridge, don't tether unless you need to because an application you use won't work over the Bridge.