Branches and brush are two different things. Obviously a the minor differences in size don't allow either of the bullets to penetrate through branches with any noticable difference in the results of the two, but small twigs yes. Both bullets carry varying amounts of momentum and are thus deflected, or have a resulting change in motion, dependant upon that momentum. Two equal size twigs with identical inertia will affect the two bullets differently. Yes, if you put a branch 10 feet in front of you and 90 feet from the target, neither bullet will likely hit the target. However, if you put the branch 10 feet from the target and 90 feet from you, the bullet with more momentum will be less affected by the inertia of the branch and thats a fact. The resulting trajectory wil differ for both bullets, but more-so for the lighter bullet and therefore the heavier bullet is preferred.
Regardless of what your beliefs are, for close range a "heavier" bullet is better, especially if there is the off chance that your bullet might go through some unexpectied foilage along its path before getting to its target.
On a side note, I would never condone shooting through brush, regardless of the circumstances, but sometimes you are shooting at a moving target and its going to happen unexpectedly. Heavier bullets are the easiest way to minimalize the affect or items in the bullets path at close distances.