Using the OP's "logic" and giving it a twist.
The OP's example is two bullets of the same weight traveling at different velocities...and from that he concludes that speed is everything...
...so let's flip that on it's head...
5.56 NATO - A 55 grain bullet traveling at 3240 fps
vs.
50 BMG - A 700 grain bullet traveling at 2978 fps
Per to OP's assumption, the 5.56 should be a much more devastating choice given it's blistering 262 fps advantage over the 50 BMG...
Ah heck, let's throw one other in the mix...
7.62 NATO - A 168 grain bullet traveling at 2650 fps
(now this just
has to be a poor performer when compared to the faster 5.56)