I haven't noticed any comparison between the average NFL lifetime of a running back and a defensive end, but the number of years a selection is likely to be available to help the team is much greater with the defensive player. Cowboys fans have a skewed perspective because Emmitt lasted so long. As a practical matter, I agree with Mick Green that usually a top flight defensive end is of greater worth to a team in the long run than a top flight runner.
With repect to Mario Williams, he may or may not pan out, but I think Jason Hatcher will probably be a greater star in the NFL, even if Mario finally cuts it.