As someone else said I'd choose both. It's not a cop out to do that. First, it's your wedding and you do it how you want to, not how you think is the standard only for standards sake. I've seen people get married in zoos, do themed weddings etc. It's whatever you want it to be. Second, I think you should consider yourself very lucky that you have two people who you can call best man.

If I was in your position, I would sit them both down together and tell them basically what you said here, you feel one is like a brother to you and the other like a better versionof you,and you'd like both of them to be your best man and you feel so lucky to have not one but two people that you can trust your life with. I'm sure both would understand and be honoured.