P vs. LT

What will be the difference between them for any given tire size?

 

On average....

Ps will offer slightly better fuel economy and often have better wet traction due to softer or special compounds designed for traction at the expense of durability.

LTs have deeper tread so are better off-road, will last longer and are tougher (chunk and puncture resistance)but will ride worse. They may be significantly heavier which could negatively affect acceleration and braking.

 

P tires are SL or XL rated. LT tires are B though F load rated.
A Falken AT3W SL 265 70 17 is 4ply 46.5 lbs vs a KO2 in C load 6 ply for 45.5 lbs.
Tirerack has some info on tire ratings.
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=55