The WT-11 Bluetooth module on the ArduinoBT is pretty full featured - it can handle more than just the serial bluetooth profile (SPP).
The product webpage can be seen here:
http://www.bluegiga.com/wt11-1
The Bluesmirf module only handles SPP. If all you are sending is commands over serial, then SPP is all you need, so you can save some money.
That said, the ArduinoBT is a very nice board - with a flexible supply voltage range.
One advantage is that if you use a BlueSmirf module, you will probably have to use a software serial port to talk to it as the Arduino will be talking to the PC using USB and taking up the hardware serail port.
I am conflicted as the the best approach - if you are doing a one-off (or very few) I'd go for the ArduinoBT else I go for the external modem.
I hope that helps a little.
Mike