The Washington Post reports:

Paul said he and Cruz came “from the same wing of the party,” but that polling showed him doing better against Hillary Clinton.
According to a Real Clear Politics average, Clinton beats Paul 50-41, a closer margin than any potential Republican 2016 candidate but Sen. Marco Rubio (R-Fla.), who would lose to Clinton 49-41. Cruz would lose to Clinton by 50-39, Real Clear Politics found.

Actually, checking the site right now, Walker does a tiny bit better than Paul too. But these are pretty small differences on which to base a campaign strategy (disclosure).