Do you go for your best friend or your girlfriend?

So I am dating this girl and have been for 6 months. She could easily be the girl I settle down with, its really good but she isnt intimate(no its not that big of a deal but it would be nice to try sometimes). She has a nice family and everything is great but we dont connect on an emotional level anywhere close to my best friend. My best friend is a girl, and she is also the sister of my other best friend...go figure. I have known this family since I was an infant so its safe to ume we are close. Well my best friend is a big of a free spirit and she has had many guys and such but is nonetheless a great person. For the longest time we have been just friends and have been able to talk about absolutely anything with one another. Recently we have grown very close as she is having trouble with guys(the ones she goes after now only want and she feels emotionally used). She has always known I am a good guy and am really nice, caring, and will always be there. She has said that in the future if we are both single that we should get married. I thought it would be awkward. But I thought about it and it makes perfect sense. She is beautiful and we connect on a level that is unheard of. The kicker is this. She being the promiscuous girl that she is probably wouldnt be satisfied with me, I am not hung like a donkey or experienced. So on that level I see very real issues. I feel bad because a few days ago we laid down after a long day in the same bed and started cuddling and later we made out and it felt quite amazing. I know I am a horrible person but it wasnt planned. I am in love with my girlfriend but I love my best friend. What do I do? Do I run the risk and break my girlfriends heart to have a shot at a once in a lifetime opportunity to fall in love with my best friend or do I stay where I am at and be happy that I have found someone who loves me. A love with my best friend would be like no other but it is not guaranteed. If I stay where I am at it is a guarantee that I can be in a relationship forever. What would you do? Im pretty confused but I would really appreciate serious answers.