well, they are almost identical. The differences are:
The Spec 3 has:
a 1 piece blue anodized chassis, that is a bit longer than the MT2.
a plastic front upper chassis brace
a plastic rear uppper chassis brace
a lay down steering servo
a slightly longer drive shaft
besides that they are the same (somebody let me know if I missed anything!)
all of the parts are interchangable except the driveshaft and chassis.
check out this thread to see some serious mods to these units!