Oh, boy. I was in Lahore too long ago to remember much. It was green and nice.
But Karachi will eventually take over Lahore by neutral observers one day, simply because Karachi does have some old historical buildings, a cosmopolitan feel (think of the amazing diversity), theater/culture scene is not that bad already, cuisine already probably more diverse than Lahore, universities are not too bad already and new ones are not hard to come by in the most-literate city of Pakistan.
Finally...the beaches! There are so many potentials of beaches around Karachi and its adjoining areas that Karachi will eventually be on globe trotters destination for year-around fly-in and cruise destinations.
Perhaps today Lahore has the slight upper hand. Only slight. But if people vote by their feet then Karachi is already the winner.
Come, say 2020 and peace all over Pakistan, Karachi will truly become the New York City of Pakistan while Lahore...very nice, very charming, very historic, will be equal to, say, Memphis, TN or some other inland cities.
PS. I am from Karachi, by the way.