or A solves the wrong problem in the wrong way and you ultimately are left with nothing.
at least B is likely to make progress in the right direction. the OP did say B's solutions were acceptable. if acceptable is true then i see no reason not to hire B. you get the solution you want with a good teammate who will eventually be as good as A technically but with a whole bunch of other goodness.
note that team instability is the largest downfall i've seen in the handful of MIT startups i've witnessed. it's crucial that people work together. i'm married to my startup partner, so working together is one of the first things we ironed out. if you're committed to that, then you can give strategy and execution an non-distracted chance.
at least B is likely to make progress in the right direction. the OP did say B's solutions were acceptable. if acceptable is true then i see no reason not to hire B. you get the solution you want with a good teammate who will eventually be as good as A technically but with a whole bunch of other goodness.
note that team instability is the largest downfall i've seen in the handful of MIT startups i've witnessed. it's crucial that people work together. i'm married to my startup partner, so working together is one of the first things we ironed out. if you're committed to that, then you can give strategy and execution an non-distracted chance.