I'm pretty sure aerobic exercise is better for the brain than strength training. Aerobic exercise increases oxygen flow to the brain and it directly improves creation and survival of neurons. Effectively it acts as a drug for your brain.

It has the additional benefit that it alters your mood, aerobic exercise makes you happy :)

Strength training may boost testosterone, but the effects of aerobic exercise is well established. Several comparisons have been done between aerobic and strength training and the conclusions have been that strength training have little effect on executive function.

My source for this is the book "Brain Rules" by John Medina [1]. Also check out his presentation on exercise [2] and his Authors@Google talk [3].

[1] http://www.brainrules.net [2] http://www.brainrules.net/exercise [3] http://www.youtube.com/watch?v=IK1nMQq67VI