Can Anyone tell me if there is a major difference between Celestion GT12T-75 watt speaker and Vintage 30's, other than the obvious power handling difference

is the tone much different between the two?

and i've got a 6505+ head, with 4, 8 and 16ohm switching, but how does this affect the cab, as im only going to use it as a half stack, what impedance should i get on the cab?
They have completely different voicings.

most people like to use 16 ohm cabs. Setting your head to 16 ohms uses more windings on the output transformer which results in a bigger low end. Sometimes its more noticeable, other times it isn't.