Full disclosure: I have never run either the AC100 or the Western States, and don't plan to, but the trails they're run on are fun and I've run much of the AC100, and getting on toward all of the Western States in my quest to "connect" from Nevada to the Golden Gate Bridge. And after Mike and I parted ways I wondered which course is harder? The best way to compare might be to look at any runners who ran both.
In 2015 there were six such maniacs. Despite that the AC100 took a median of 9.1% longer for these six to finish, they finished a median of 5 percent better in the rankings in AC than in WS. The obvious explanation for this is that although WS100 has a more competitive field, AC100 is more difficult. (Interested in this stuff? Check out Dick Collins 50-miler scatter plot of age vs time. Difference for men and women!)
So: Jeff Giumarra, Matthew Menacher, Jorge Pacheco, Mauricio Puerto, Franco Soriano, and Mark Tanaka, this Bud's for you!
The total elevation gain loss for WS of 41,000 is less than 46,390 for AC, and AC is higher on average. The page I got this from has a number of other profile comparisons, many of them done by Chihping Fu.