There's really no choice here. This doesn't seem like it's a sure thing....if it is why are you tempted by some fantasy? Even though this person may have strong feelings for you it's not fair for you to choose them if you don't love them in the same way. It sounds to me that you don't. If you did, you'd know it, there would be no guessing, no choice to be made. If you care at all for this person you won't play with their affections in this way. They deserve someone that will love them as deeply in return. How can they find that love if they are wasting their time and efforts on you? Your only option is the fantasy...go for it. It may not work out but at least you won't have any regrets or wonder later on what could of been.