nvidia outsells amd about 2 to 1 and they also "seem" to have a higher markup on their gpus so their gpu department nets a little over twice the amount per year than amd does with their gpu department. this in no way is a real indicator of who is better, they both are generally very close as far as features and performance.
that a loaded question. for value for low end and entry gaming gpu amd kills nvida. nvidia higher end gpu are faster in some game. nvidia is like intel now they dropping newer tech faster then amd. few years ago when amd was ati ati used to drop newer tech faster then nvidia did. the issue now is amd does not know if it a cpu/gpu or a foundry.