If u are travelling for the first time ( Specially to North America ) try avoiding a transit in the US. They will do ur immigration and background questioning there.
Try taking the direct PIA flight. It will give u the Max baggage allowance and its direct.
Now i know PIA is not the best carrier, but it will def be the best in terms of timings. Second option should be Dubai to Toronto- Direct.
Avoid the hassle of US customs/ Immigration and excessive questioning.
Boston bombings are taking place as i write this post. (very unfortunate and lets hope it doesn't turn out to be some stupid Pakistani).
Now having said that, the US airports will go on special alert for the next couple of weeks/ months. Avoiding them will be a good idea.
Good luck and safe travels.