I think Mosley's speed would give him the edge. Buddy would probably be one step behind for most of the fight but he'd definitely have his moments. Mosley's power would also keep Buddy honest. McGirt would win some rounds but I think Shane wins by a 116-112 type decision.
Mosley would win a close decision on the basis of appearing to outwork McGirt by throwing rat-a-tat combos that basically land on the flanks and do very little... kind of like the way he won the second Hoya fight... Evryone's going to say "MELDRICK TAYLOR!!! BLAHBLAHBLAHBLAH"... save that shit... watch that fight and tell me honestly: 1) was McGirt as good at that point as he was later on as a Welterweight when he was taking Simon Brown apart and putting up a very respectable showing against Whitaker? 2) really watch what Taylor is doing and then watch a Shane Mosley fight... they are both black guys with fast hands, the similarities end there... seriously, how can you watch that and say "yeah, Mosley does shit like that all the time"? 3) what real good defensive fighters did Mosley ever have an easy time with exactly? Mosley wins because he'd throw a lot and McGirt's counters wouldn't seem like they were coming often enough... but anybody who thinks Mosley shitkicks any good, smart welterweight ever is nuts... he was not that great, he essentially had one guy's digits