I vote for the new truck. We just traded from an 06 3500 Mega to a 15 3500 Mega.
We bought the 06 used with 90k and ran it 5 years and put another 100k on it, while it was a nice truck and very reliable, it doesn't hold a candle to the 15 on refinement, quietness, and comfort. Even when we first bought the 06 I always felt that fit and finish on that truck was not the greatest, and honestly felt that my 99 was built tighter and more refined than the 06. Probably my biggest complaint was that the drivers door on the 06 leaked air constantly, and no amount of tweeking or weather stripping could fix the problem. Every 06-07 vintage Mega in this area I know of has the same problem. Mechanically it was a very sound truck as to be expected, other than a set of injectors and general maintenance, it was always a very reliable truck, except for the very end with a 4wd electrical gremlin. In regards to the transmission, I had 190k on original, but I maintained it religiously with oil change and band adjustment every 30k, and the truck was bone stock.
The 15 is a lot more quiet, better fit and finish in the cab, no air noise what so ever, and just an all around nicer truck (too be expected right, its new). I got the Aisin, SRW, with 3.42's and it is getting slight better mileage than my 06. Just checked it the other day, and for the 4200 miles that are on it, it has averaged 19.5 ( not hand calc'd) where my 06 would do about 17-17.5, and DEF usage has been nil. Took 2.5 gallon jug at 3000 miles to bring it from half to full.
All and all, I would highly recommend the new truck, we love it.