I have worked and lived in both places in past two years. Lahore, of course, has better infrastructure these days and is much cleaner and safer than Karachi. But it is losing its vibe because of a majority of its new development projects is being carried out the expense of its cultural identlty
Karachi, on the other hand, is more culturally/ethnically diverse and street food is palatable (more so than Lahore, with some overrated places in both cities). The city definitely has its own appeal with greater and better recreation spots and sea breeze feels heavenly in summer evenings. It's a great city to live in only of the security situation was better and if the infrastructure is developed to an extent that it can support its population (which it currently doesn't at the moment).
And since I have worked there, I have considered relocating to both cities. While it's conceivable to do so in Lahore (if ever needed), it would be a more difficult decision to do so in Karachi. Primarily because it will take me farther from most of my friends and relatives and secondly street crime in Karachi is what dissuades me. I myself witnessed a snatching incident every single month I lived there (while not even once living in Pindi/Isb most of my life). Although, I must say Karachi is a sort of city I wished I would have spent some years of my childhood in.
Having said that, being a Pindi boy has its own charm