With Dexron-VI coming into the market, they may have changed their bottle to how it is now. All of the other companies have changed theirs to where it states "Not for use in vehicles requiring Dexron-VI."
Yes, Dexron-VI is the replacement for all DexronIII applications. Essentially, Dexron-VI is a far better fluid than conventional DexronIII, but synthetic DexronIII ATF may be just as good, if not better, than Dexron-VI (not trying to start a debate, it's been beaten too much already).
For you, I would say to go with whichever is cheaper and more readily available, Mobil Dexron-VI or Mobil 1 ATF. You're going to get an improved ATF, over conventional DexronIII/Mercon, regardless.