What are the differences between the normal 2007 Enduro frame and the S-Works one? As far as I can tell the only difference is the S-Works frame is made of M5 alloy which gives a slight weight savings.

Here is what I found on M5 on Google:

ABOUT M5 Alloy
M5 was developed directly from the phenomenal results achieved with E5 Alloy. In fact, the base aluminum and alloying elements are the same as E5's, but tailored to meet the needs of off road frames.

Specialized engineers have a clear objective - design the lightest mountain frame possible. What they learned with E5 lead them to the M5 alloy which shares the outstanding strength (and hardness) and elongation characteristics of its cousin. Specifically:

M5 has exceptionally high tensile strength so less material can be used without sacrificing strength requirements.
M5 retains substantially more of its strength (hardness) after welding than other alloys, again enabling less material use while maintaining high strength.
M5 has fantastic elongation, which means it can be radically manipulated, allowing a truly optimized tubeset.
Where it's found (in the line):
M5 is utilized in the S-Works hardtail and S-Works where cost is no object and ultimate performance is.