Having just swapped to a P-rated 33" tire from an E-rated MT, I say go with the P-rated if you don't "need" the 10-ply or if the tire you want isn't only offered in the heavier load rating.
I had the Cooper STT 285/70/17s for ~8 mths. My avg MPG was right around 16.5-17 with mixed city/highway. Although I didn't necessarily mind it, the ride was definately more harsh/firm (of course). Also, my truck took a decent hit in acceleration but that may not be as big of a deal on the V8 (mines the V6) with the extra torque.
I have recently swapped out to a P-rated 285, Toyo AT II, and love the ride. I feel like i'm getting the benefits of the larger OD tire without as many of the negatives. For reference, the current tire weighs 48lbs while the STTs were 58lbs. My calculated mileage is already back up to 18.5-19 MPG with the same mixed driving. Acceleration is MUCH better as is the ride. Granted this is all common sense but just wanted to give you my experience. Also, keep in mind some of these changes are due to going from the MT to an AT too. In summary, I would say if you worried that much about the negatives of 285s stick to the Ps. Either way, 33s are definately worth it.
