The VTF-2 seems like a giant-killer - particuarly at the $250 price point that a used one can be found on at Audiogon...but the Dynamo seems to get its share of respect as well.
Note that there are several different iterations of the VTF2, so that will affect the outcome. At 250 bucks though, I'd say any of them would be a great value.
On the other hand, the Dynamo is a great music subwoofer; however, its output will be somewhat limited compared to the VTF, so if your room is large, or you like to blast it, the Dynamo might not be the best choice.