Perfect Storm explained it well.
Setting a certain browser as default depends usually on your dataplan and network accessibility.
On my BIS device I usually take the BB-Browser (BIS APN on unlimited dataplan) and switch to WiFi if my private WLAN network is available (for example at home) because its faster.
I would have to pay for traffic using WAP, so I almost never use it.
My default browser on my BES device is also BB-Browser (connection via BES).
Hope it helps a bit.