I read a test between these two, can't find the link sorry, and there was actually neglible difference between the two, although AS5 pipped it by a degree or so iirc. As pre applied stuff goes, MX1 seems to be really quite good. However, AS say that AS5 is only properly burnt in after 200 hours, and I don't know if the review did that to see if there was any difference after that.
Theoretically speaking though, the mega silver content of AS5 should be a better heat conductor.