I owned a '10 Stumpjumper Comp carbon ht and recently replaced it (after it was stolen) with a '12 El Mariachi 2 complete. I also tested the '10 Stumpjumper Comp alloy before buying the carbon version. Here are my thoughts:
1) I did not at all love the feel of the alloy Stumpjumper. IMO, it feels harsh over bumps but also not particularly laterally rigid. I bought the carbon version mainly because it felt so much more responsive under pedaling power. In general, I don't think alloy 29er ht's make sense or at least I've never ridden one I like compared to either steel or carbon. (Although I do really like my Selma that is Scandium with carbon seat stays).
2) Comparing just geometry, I like the El Mariachi better. IMO Specialized tends to make their top tubes too long, making it harder to properly weight the front tire when cornering. Salsa geometry tends to feel perfect to me (comparing XL sizes in both). Also, I thought the Stumpjumper chainstays were slightly too short, and the bb was a little too low. In general, with a 29er, I think putting the bb as low as the stumpjumper makes the bike feel less agile in technical situations. One nice thing about the El Mar frame is that you can adjust the chainstay length to your preference (though even the shortest possible choice is slightly longer than the Stumpjumper).
3) The El Mariachi is not the most lively feeling steel frame I've ever ridden, but it feels pretty good to me, at least in the XL size. It also feel relatively laterally rigid for a steel frame. Comparing steel to carbon hard tails, I think that carbon feels a little better at damping high frequency vibrations but steel feels much better on big hits. Overall, I'd choose carbon for XC racing but steel for all day riding. However, I've done a couple of races this season on my El Mariachi, and I was very happy with the performance of the bike (the performance of the rider could use improvement
In summary, I'd choose El Mar over alloy Stumpjumper every time. If I was choosing between carbon stumpjumper and El mar, it would depend on whether I wanted a faster bike for racing vs. a better bike for all around XC riding.