In an Excelsior 550 is 26.5" dia------600 is 28" dia That 1 1/2"---pretty much difference! The 750 is 31.60"
Omitting that the tires in question are Excelsior's may or may not have any bearing on the question,depending on which size you go with it only effects the height by 3/4" of a inch,,I still submit it makes very little difference. I don't think any of us could detect one from another if they are not side by side,,, I guess it boils down to what you want,,personally I like a rubber rake some guys don't care for it,,it's a personal preference. HRP
Depending on th tire and wheel combo a 7.00 may look a little meaty. I would probably run the 6.00 on a roadster.
It also makes a difference if it is a T, A, or B roadster...I like smaller front tires on smaller cars...balance is key...fung shway