Class 4 & 5 gas powered trucks

I was wondering if anyone had any experience with the newer class 4 &5 trucks with the gas motors (f450, f550, Ram 4500, Ram 5500)? I know it seems like there are more and more on the road. I test drove a 6.4 hemi Ram 4500 crew cab 4x4 with the Aisin 6 speed automatic a few days ago and it impressed me more than I was expecting, but it also was not pulling a load. I was wondering how these would compare to the v10 in the F450 and F550's with the 5 speed automatic? I know a gentleman that has the Ram 4500 and loves it hauling cattle, hay, and equipment. He says it doesn't pull quite like the 6.7 Cummins he had before that, but it is close. And cost several thousand less and is getting better fuel mileage on average with cheaper fuel. Not really in the market for another truck right now, but getting several miles on my 2003 Duramax and just like to keep options open if it looks like it might become repair prone. Those trucks with the gas engines look to have higher towing (and much higher payload due to higher class truck) than my 3500 flatbed dally.