I got one of each for my printer and then had to put them together. The E3D V6 I got uses a small teflon tube inside the heat sink and a tube retainer at the extruder interface that sticks up about 1 mm.
The first problem I ran into was that collar of the hot-end was too wide for the groove in the BullDog's foot. I used a small file to put a couple flats in the E3D mounting collar so it would fit into the slot. The next problem is that the heatsink fins on the E3D extend over one of the screw heads that secure the foot on the extruder. I used a round file to cut a groove in the heatsink fins on the hot-end allowing access to the screw head. Finally, when it was all bolted together, the hot-end wobbled. The problem is either the BullDog XL foot is a little off or the E3D V6 collar is a little off or both. I found that if downward pressure were applied to the top of the hot end it would stabilize. So I started looking for spacers that would work. It turns out a standard, cheap, zinc plated 1/4" flat washer fits closely around the plastic tube retainer in the hot end and is just the right thickness to press against the hot-end and lock it down tightly. I chucked the washer in a lathe and turned it down to just a hair under 16 mm dia. and assembled the extruder/hot-end and it works perfectly.

There's no need to pay $30 for a new foot for the BullDog XL. Just get a washer and reduce its diameter with a grinder, file, or lathe. Precision isn't necessary.