AMSOIL Signature Series ATL for your transmission. One has to consider how often they want to do a drain and fill their transmission and how it will handle the heat. Yes dexron VI is the specification but you have to remember that each brand out there that has the specification pretty much just "meets" the spec.
Going with a full synthetic, you have the benefit of the fluid being able to keep the inside cleaner, cooler and resist oxidation at higher temps.
Again, keep in mind that most don't mess with their transmissions until they start to have issues. If you keep the fluid changed out every 30k miles then you will have a long lasting transmission over the ones that never change it until it hits 100k and feel like it needs to be serviced.
I've had AMSOIL ATF and now ATL in my 2002 silverado's transmission since 10k miles. I purchased it new and switched it over at 10k. Since then I changed it back out with a pan drop at 77k and then a full swap over to ATL at 89k. Then ran it to just recently at 189k and had the fluid sampled. It came back just fine. This is with towing a 4k lbs 10'x 20' box trailer. Transmission temp gets to 197° F when I tow. I also pulled the pan and replaced the filter and the pan looked just like it did back at 77k and very little on the magnet.
This is just me but why go with a lesser cost fluid that is not full synthetic on something that you normally don't worry about until it breaks. That is for the most part what the average person does. You paid big money with your hard earned cash for a vehicle, treat it well and it will treat you well.
Anyone interested in AMSOIL transmission fluid. PM me what you want and how much you need and I can send a quote delivered to your door. All GM-Truck members get a discount. If you are a preferred customer than you already get the best price and from now until the end of the month PC members get free shipping on orders over $50.