Use the lowest cost one you can find that still meets the specs. I use the Walmart ST ATF. if you want to spend a little more per gallon, then get the Pennzoil ATF. In the large GM trannys, there is very little decompostion of the fluid in general service.
You might want to splurge @ walmart and get the ST Dexron 6. it is $1.00 more per qt, but it is synthetic and better than Dex 3