i would expect a one-stop flight thru a non-hub airport to use the same aircraft all the way thru more often than a one-stop flight thru a hub airport. granted, there are other 738's at LGA, but i wouldn't expect them to change aircraft unless there was a severe delay on the YYZ-LGA leg. i wouldn't worry about having to run from one concourse at LGA to the other -- if they change aircraft, i'd expect them to do it because you are arriving at LGA from YYZ long after the LGA-MIA leg has departed.....

now, will they let you stay on board? that usually depends on the FA crew. if the FA's are staying on board, you can stay with them. if not, you have to deplane and reboard with the local passengers.

but, i guess what i'm saying is, book it. it's more likely to get you to MIA than a true connection at on of the hubs....