EP has an 18% +/- stronger additive package, has double the amount of PAO, and is a more shear stable product than regular Mobil 1.
For "extreme service", you might also consider Mobil's 0W-40 or mixing a 50/50 blend of 5W-30 EP and 0W-40 for a very robust oil for long-term hauling. (0W-40 has a stronger additive package than EP, which is better than regular Mobil 1, which is no slouch.)
Right now, EP and the regular version are the same price ($26.68 per 5-quart jug) at Walmart and there is a $12.00 rebate per 5-quart jug available from Mobil (up to two jugs with a max rebate on Mobil 1 being $24.00). 0W-40 is also $26.68 and is a world class oil; EP is underrated IMHO and is what I use in my vehicles.