Honestly I think 600 would work well for you but since you have both carbs and it is one of the more simple switches you can do I say try both and if you really find that one works better then the other then use that. If both of them work the same and feel the same then use the 600 and save some gas. Thats my two cents
Definitely go with the 600 if yours is a street light bandit. Vastly better throttle response and as the guys say above, the smaller carb will probably still have longer legs than your engine on the top end.
The 750 will prolly give ya a wee bit more power but at the sacrifice of throttle response.For lowend response go 600.
Being as you have both do like said and swap um out and see what ya like.You may find the 750 to not be so bad reponse wise and find a little added power or you may hate it.But try um both.
The difference in your top end using a 750 as opposed to a 600 will be nominal unless your motor is a high winder. You will not see (seat of pants) any difference between the two. The 600 will give you virtually the same performance with much better throttle response than the 750.
But if you have both try them - see how you feel about the two.