I have the 8.1L Yukon XL and around town I get about 10-11 MPG. 3:73 gears and about 32-33" tires.
When I tow the 94' 205, I get around 8-9 MPG at 75 mph. It tows it quite well.
When I got mine, I looked at the 6.0L and it only felt "OK" but not really "peppy". The 8.1L is a lot more snappy and more fun to drive. Especially with a vehicle that heavy. Generally speaking, you will probably lose about 1 MPG with the 6.0 vs the 8.1 but in my mind, it's worth it!
There are very slip pickings when it comes to aftermarket parts on the 8.1.....much more on the 6.0. But for general repairs, OEM parts, no problem.
It does drink oil so don't be alarmed. I go through about a quart every 2500-3000 miles and it's been that way since I bought it (approx 20,000 miles).
Here's the only pic I have right now....(dead boat battery)