Yes it is apples and oranges, but for reliability and longevity the 6.0 wins hands down. Granted the 8.1 has more grunt, but 6.0 has enough to get it done too. Your overall cost of ownership will be much less with the 6.0 all things considered. Mpg, maintenance, reliabilty and repairs etc.
And you'd never get over 8.5mpg with an 8.1, 7 was pretty much the norm in 2 different C2500HD trucks had.
Ohhh and we haven't even talked oil consumption yet.
Everyone I've known with an 8.1 has had no reliability issues at all. Just one guy with oil consumption, and that was an early one. It wasn't even that bad, you're supposed to check your oil level anyway no matter what you drive. Most of the 8.1 trucks I found for sale had well over 200K on them and still going strong, so they are long lasting too. As for mpg, it's not a big difference between the two engines. It's not uncommon to get 11-12mpg from an 8.1 in warmer weather (drops when colder) and obviously less mpg while towing. Obviously, a 6.0 should bet better mpg unloaded, but it's only a couple better from most reports. My wife's first tank in this one was 13mpg. Both engines get about the same mpg while towing, one just has a lot more grunt while doing it.
Really didn't matter which engine we got though, for years we've been getting 7-10 mpg out of our old worn out Burbs and been dumping a bunch of oil into them too, so even the worst case 8.1 scenario would put us in a better place than we were before. But we got this 6.0 Burb, which is just fine too. It was clean, had bench seats, only 89K on it... and it was within our budget. And above all, the wife likes it!