"P" tire designation vs. "LT"

Thanks for your quick response. I guess the follow up question should be about tire selection when I replace them. I know this will probably open up a wide variety of choices but maybe not. Is there a preferred tire brand and should I go to a 17 inch rim verses the 20 inch rims that came on the truck.

I've heard that a 17 inch rim is preferred for towing. Is this correct? Thanks again.