Disc Brake Conversion Poll

I have the opportunity to purchase the disc setup off a diplomat or an aspen. Will the diplomat likely have the larger rotors?

If so, is the benefit of the larger rotors worth the purchase price of new 15" tires? I am broke and my 14" tires have less than 6000 miles on them.

I really can do it either way but I am curious how much improvement there is in stopping power with the larger discs?