I was comparing Helvetica with San Francisco, and I think San Francisco is much more legible. I thought it was narrow than Helvetica, but it actually takes up the same horizontal space, it's just the the letters are narrower and there's more space between characters for the default tracking. SF is not as pretty as Helvetica, but I think that hurts helvetica's legibility. The consistent curves and wide letters look nice, but are harder to differentiate. San Francisco looks much more utilitarian.