I'd say go w/the MRT. Like everyone else said...I've heard good things about it also. I got a MAC because a coworker sold it to me w/the MIL's for 100$. That's a steal. :D
I highly suggest staying away from the SLP stuff unless you like the sound of F'bods. It highly resembles there sound and IMO takes away the true muscle car tone that mustangs have always had.
Thinking logically and economically...which one would you prefer? Which one is better for your money. I know that it's 2 totally different formats of superchargers, but I'd like to hear some opinions on this.