The AC adapter is much faster for charging.
I think an accurate analogy is a trickle charger for your car battery vs a quick charger. The USB buss doesn't provide much power and charges very slowly. I typically fully charge most completely discharged BB devices with an AC adapter in 30 minutes.
In SF, there are street sellers at 50 UNP (Civic Center) every Thursday (and sometimes Fridays.) There is a vendor who shows up regularly with cell phone and BB accessories. You can get an AC adapter for your BB for $11. They cost $29.95 usually from stores and e-commerce sites.