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That's a strong argument one would have to defend to say these are the most influential action movies. The two movies are nothing alike at all and have had their own indelible impact on movie history.
I remember after Mad Max 2 (Road Warrior) came out there were so many movies that were jumping on the bandwagon copying it's gritty style and look. The 80's was full of shameless knockoffs and parodies. Even the G.I. Joe toys I had were influenced by the movie with Road Pig being a blatant ripoff.
Road Warrior (MM2) was certainly popular, but I don't think it was anywhere near the summer blockbuster that T2 was. I mean even T2 was selling toys to little kids who should not have seen it because, after all, it was rated R...right? I just remember that film being in the mind of the culture for years after it came out through such mediums like arcade games. T2 is definitely a more polished, superior sequel to the original at least with the FX, and a big budget movie as opposed to MM2. Yet, off the top of my head, I don't know how influential it was, compared to MM2, at influencing other movies except for influencing other Terminator movies. I would be interested to hear anyone's thoughts on what movies they think were a direct influence from T2.
If I was stuck to choose one to watch I would choose Road Warrior over T2 as I honestly don't know how many times I have seen it. It was a unique piece of creative film writing, extraordinary characters, and a rare use of original-style fabricated vehicles to the point that they themselves were almost characters to the story line. The plot is something based more in reality in that it could almost be conceivable having bandits fighting for fuel if the world were stopped by war.