Ah, But Which One Am I?: The first copy of Mysterio on your force costs 70 points and begins the game on the click #1 that appears after two consecutive KO clicks. Additional copies of this character on your force cost 15 points each and begin the game on the click #1 that appears after the KO. Mysterio is considered 70 points for all game effects except victory points.

Smoke and Mirrors: Mysterio can use Incapacitate and Smoke Cloud. When he uses Smoke Cloud, after actions resolve you may place this character and any other AS #026 Mysterio in each other's squares.

Holograms and Hallucinogens: If no other friendly character named Mysterio has used Outwit this turn, Mysterio can use Outwit.

Can I choose to just play the 15 point version of Mysterio?
No. The first copy of Mysterio you place on your force must be the 70 point version.

Well, can I play more than one 70 point version?
No. Additional copies of Mysterio must be the 15 point version.

What exactly does Ah, But Which One Am I? mean when it says, "Mysterio is considered 70 points for all game effects except victory points."?
It means that during the game, you (and your opponent) will treat all of your Mysterios as if they cost 70 points. For example, a character that cost only 50 points could not use Mastermind to transfer damage to Mysterio, or a character using Mind Control would take 1 click of damage if he controlled two of them (since 70 + 70 = 140). Basically, you can't use effects that check for point values to try and figure out who is who.

But when the game is over and Victory Points are scored, the various Mysterios are only worth whatever you actually payed for them in the first place to put them on your team.

Will any of these effects work with other versions of Mysterio? Like the SR from Web of Spider-Man? Or the REV set from Ultimates?
No. These specific effects belong to this specific Mysterio. Only other Mysterios possessing these same effects will be a factor. Just because your WS Mysterio uses Smoke Cloud does not mean you get to swap places with this AS Mysterio.