I vote for building your own. I find most premade kits are more boo-boo kits than something you could use in a serious situation. (Not that there's anything wrong with having a boo-boo kit as well, but band-aids don't do much to serious injuries.) All pre-built kits I and my girlfriend (medical professional, though works in mental health, not trauma) have looked at have been lacking in at least some way.
Building your own also gives you more familiarity with what is in your kit. Kneeling next to a bleeding, unconscious guy is not the right time to start wondering about what, exactly, does this store-bought kit actually contain. (Guess how I found that one out? And sure, I looked over the stuff in that kit a couple of times before putting it into my car, but as it turns out, that wasn't quite enough...) And last, building your own kit lets you build one that suits your skill level and preferences in first aid gear.
It's a Jeep Patriot thing. You wouldn't understand.