KE is the better of the 2 for ESTIMATING penetration. Another thinng to add to the equasion is the effects of speed OTHER than that of just KE or momentum.
Simply put, faster things cut easier.
We have all seen or heard of the fixed blade test. try pushing a fixed blaed (cut to tip) through leather, and a replaceable blade (say a Thunderhead) through leather. it seems VERY hard to get that T-head through the leather...almost to the point of bending the arrow. Why do they zip in one side and out the other on a deer (when palcing the shot correctly)??? SPEED Why can you take a bicycle spoke and push (HUGE amounts of KE and momentum here...say 200 lbs of you moving at 1 fps) it along the grass and nothing happens...but a weed wacker goes through grass like butter? SPEED.
That said, I AM going heavier this year for more of a quiet bow...I' ll be droping from about 300fps to 280 or 270. (125 gn heads vs 100s, and CX300 vs CX200 will be adding over 50gns).
I' m not a speed demon...but I think some go TOO far in looking at ONLY momentum and to a lesser extent KE.
FACT #1 you CAN' T bow a bow quiet enough for a deer NOT to hear
FACT #2 You won' t make a bow shoot faster than a deer can hear.
the best you can hope for is a compromise...quiet enough that the deer doesn' t CARE...and fast enough that IF he DOES care, he can' t move TOO far out of the way.