My old 1997 F150 XL came with LT tires and my 2007 F150 XLT came with P rated tires. The LT tires have stiffer wall and the load rating is E which can carry more weight.
The P tires have softer wall and load rating is B (I think) which carries less weight. If you load your truck up to the limit all the time than I would go LT tires. If you don't carry lot of weight day in day out than P tires would be sufficient. I personally like the P tires because it gives softer ride. I generally carry 600-1000 lbs daily and I have no problems with the P tires. This is just my 2 cents.