The second one, but I think I'd like it even more if there was some ground underneath the tank to make it look less like it's flying.
Looks good. Other thoughts would be to have the ground extend out of the border as well and/or shrink the mushroom cloud slightly and move it to the top/right a bit.
In second thoughts im not sure on the logic of the logo, The explosion is more like an Atomic Ash Cloud than a explosion a tank could create.
Thank you guys! I will experiment today a bit more and post the results. Your help is appreciated! We couldn't decide I know, but this looks the best from all the explosions we have tried