if ur just using what u have in ur setup I think the 120amp alt with either the red or yellow should be fine.
I think the red has longer warranty because it's more reliable as a start up battery and most people use the Yellow top for audio, so they know the red one will last a long time cuse it's not used under alot of stress. So warranty's longer.
Hence why yellow warranty only 1 year free replacement, cuse all the stress they expect people to put it under.
btw (Firebirds come stock with 100+ too, my bro's is 125 if i rember correctly. I think most import's have lower output alternators).