I know personally that 19 inch tires are loads more expensive and rare than 20 inch tires. you should really just go with the 20 because of the availability of products. I would hate to end up with a flat somewhere, and the only thing the nearest tire place keeps in stock is 17, 18, 20, 22, and 24 inch tires (like so many places do...)
Dub it out...
tha's why i keep a 16" OEM alloy wrapped with 205-60-16 rubber in the back