I poked on the interwebbs a bit and have no idea what the difference is. I just go for the Supertech (generic walmart brand) Dexron-VI. As I understand it, the standards required by the Dexron-VI licensing are very stringent, worlds ahead of the Dex/Merc that anybody can do anything with nowadays.
Use the Dexron VI; even if you can get old stock. I went through this when I replaced the fluid and filter on my 98 Buick. The VI is better, you can read up on the improvements and it can be mixed with what little bit of fluid remains in the trans.