Hey Ikle, I really like the Dex VI but if transmission internals are factory issue, there is a very good chance that the band and clutches may already exibit glazing/slightly burned friction materials which combined with the newer fluid might induce some slippage that won't be there with Dex III, if trans is fresh or recently gone through(even if only for inspection) I wouldn't hesitate, the newer fluids deffinately hold up better to the increased heat that we tend to inflict onto them, but it could send a glazed band over the edge which then would require a reconditiong along with all new fluid again(Dex VI isn't exactly cheap either)
If trans is known good, absolutely go with Dex VI, if not or is questionable, then it might be wise to stick with Dex III to avoid potential trouble.
Good luck,
Kevin.