I have played both machines, and have had dozens n dozens of games on both titles, and in my opinion, Avengers leaves Spiderman for dead 10 times over.
Spiderman is a nice looking game, but Avengers is better looking.
Spiderman is a reasonable player, but Avengers is a better player because it has more agles and has more depth.
The toys on Avengers are better, and the general illumination and insert lighting on Avengers is better than spiderman.

Having a brand new in box experience also leaves spiderman behind. For only a small amount of extra money, Avengers is the smarter buy.
Spiderman is one of Sterns better players, made during the cost cutting days, but compared to Avengers, it is very basic.

Goodluck with your purchase.