Don't be fooled by to the "Gun Slingers"
What is a Gun Slinger? The guy that can be found, usually within eye shot of the mirror, who is throwing around impressive amounts of weights with absolutely no form or proper technique. Gun slingers usually have very narrow-minded, and usually wrong, ideas about physical fitness that most get out of a half-a$$ed read Men's Health mag.
Building muscle is based on two things; tearing and healing muscle fibers. That means the best way to gain mass is to most effectively breakdown the fibers and provide the body with enough protein and calories to do what it does and repair those micro-tears. Those that say you must bench-press to gain size clearly don't know biology. While bench-pressing is an effective way of building muscle, heavy lifting (anything more than 60% your max) is usually counter-productive because: 1- you hardly ever use the muscles you're trying to build and typically add "cheat" muscles to lift. 2-you typically only do a few reps before you can't effectively lift the weight. 3- you are more prone to injury which means you have to take time off and potentially loose whatever bonus lifting heavy may have giving you.
By lifting lightweight, you can focus on both form and the muscles you are trying to make larger (i.e. the chest, tri's, shoulders), do more reps, and avoid injury- all leading to effective muscle fiber breakdown. Accompany this with pauses, squeezes, and slowing down your reps, and I guarantee you will gain size; this especially with push-ups because it is a complex exercise which requires several muscle groups to perform properly. I know several big guys in the army, as well as Jay Cutler (the champion bodybuilder not the QB) use this philosophy with great results.
A great push-up exercise I use is a tri level, high-rep super-set. I set up two platforms, one at a height of 2-3ft and one at 1-2ft, and give myself room to perform regular push-ups as well. Knock out a set of 10-20 on each station, starting from highest to lowest, with no breaks between stations (so 20 incline, 20 half-incline, 20 flat for 60 total without stopping). 2-3 full sets of this (minimizing rest time between full sets) will smoke you.
If you can do more than twenty on each, slow down your reps and decrease rest time (to nil if you have to) so you are hitting muscle failure around your last push-up.
So which is better Push-ups or Bench-press? It really depends on your fitness goals. If your goal is athleticism, health, or endurance related, having a push-up heavy routine may be quiet beneficial. If mass is your goal, a combination of both is great for keeping the muscles guessing (remembering eating has a lot to do with mass gains). If show muscles are your goal, I'm afraid I can't help you; just keep on slinging!!