Who do you have the most fun with....who makes you laugh more....
It's difficult when this happens.
I find it intriguing that you think you don't have as good a chance for one of them....and I bet you're wrong. Generally I've found that the one you think is less interested is just playing hard to get. Although I also find that these people make unstable (although passionate and amazing) relationships.
The problem isn't the choice though. I think deep down you know which one you want. It's the 'What if?' factor.