While there are several similarities -
The ageing sheriff - Wayne/Schwarzenegger
Loyal Sidekick - Brennan/Guzman
Town Crazy - Brennan/Knocksville
Great fighter turned
town drunk - Martin/Santoro
Hot female ally - Dickinson/Alexander
I think that it's more of a homage to Rio Bravo and El Dorado rather then a 'rip off'.
Not so much in the plot, Assault on Precinct 13 has a lot more in common with Rio Bravo. What really struck me with Rio Bravo is the jail being the main location, and wherever anyone went, they always came back there. The Last Stand doesn't have a focal location, everybody spreads out throughout the movie, and don't stay in one place long (lingering around main street the most). The Last Stand is similar in the characters (as pointed out) and the townspeople lending a hand to the sheriff. The real difference is the objective; in Rio Bravo, John Wayne had to keep a prisoner in his custody and in The Last Stand, Arnold Schwarzenegger was trying to capture an escaping prisoner.
It does have the feel of a Western, the hero defending his southwestern town from the badmen.