I don't have any experiance with the marfields, but the Longs are all you could ever want, and more. I have beat, my 30 spline longs and they have never flinched at the stresses I have put them through. I'm only running a 36, but I know they would hold up to more tire.
My buddie runs the same 30 spline long set. He broke the gear inside the hub dial, the chromo long gear, also broke, and rounded the splines on the inner axle......Bobby was curious about what happened, but had a replacement sent out the next week after recieving the damaged parts. Bobby is a great guy to do buisness with, and will recieve my money again. The lifetime waranty is what sold me on his parts, but the durability of the product, and great customer service make em worth the price.