I went through the same debate for weeks. Finally decided on the 48 for the added flexibility. I like having that extra space when friends come over or I'm cooking several different meats at the same time. Plus my friends said, "if you're debating this long over it...get the larger one." Which, I have to say that I'm happy that I did. If you have it in your budget, go for it. It'll cost you more money to upgrade to a 48" if you decide the 36" isn't large enough. Good Luck, you'll be happy with any of them.
Lang 48 Patio, Weber OTG, Weber Genesis and a Hot Coals Thermapen