Go hard for girl B. She is what will make you happy.
Keep A around as a safety net. If she wanted honesty and exclusivity with you, she would seek the label of relationship between the two of you, but she does not. If B pans out, you have the option to cut ties with A. If B doesn't pan out, you will always have Girl A, and if she's unhappy about this play,
well, she's the one who wanted to have a loosely defined relationship with you so... that's just too bad for her.
I know it's a dick move towards Girl A, but sometimes you have to be a dick so you don't get dicked around in the end.
(By getting dicked around, what I meant was: Despite this cute trust thing you have with Girl A, having that loosely defined relationship also means she's just as open to do the same to you as you are now to her.)