Are you looking to buy a new suburban? I would stay away from swapping motors if possible unless its a project truck. What I love about my 6.0 in my 2500 is that it does not have active fuel management. No switching between V4 to V8 and minimal shifting. I considered a 6.6 when shopping for mine back in Jan but the new diesels require urea additive to comply with epa. Not a big deal just another thing to do. Unless you need the power to tow and you tow often I don't know any reason to get a diesel. For those saying better mileage, you have to drive a few hundred thousand miles to recoup the savings vs the extra cost upfront. I have towed just shy of 15,000 with mine and had no issue at all. went about a hundred miles, up hills and all. even passed a few big rigs going up hill at 70mph. My gas mileage did suffer a bit but I don't tow every day. Oil changes cost more, fuel filter every 5K and diesel costs more than gas at least here in southern new england.
If you are considering used, I would look for a 2500 suburban I think 2005 or 2006 was the last year for the 8.1 big block. and other 2500 suburbans have the 6.0 already.
Don't get me wrong I would love a duramax but being at least 7K more at the dealer wasn't worth it to me.
Good luck with your search!