The only reason I can think of not to get the BigDummy is if you think it's likely you'll want to take the Freeradical off at some future date. It's not something you can drop on and off when you feel like it though. The BigDummy is bound to be stiffer, but also, it has geometry specifically designed for the long wheelbase, and it looks from the blog as though Surly has put considerable time into getting that right, trying different forks and wheelsizes. That alone would be enough to swing it for me.