14 inch vs 15 in

I need to replace my current 14 inch SS cragars on my 1973 Mustang Mach 1 & I am considering buying some Magnum 500 Mustang mags but am not sure if I should change to 15 inch or stay with the 14 inch.It would be less costly to stay at 14" as the tires I have are only 2 years old & have all the tread as I put no miles on the car.But I know the 15' fill the huge tire bay much better.
Please advise the benefits & Drawbacks of changing up to 15's
Regards..
HRL