I was told in a dirt bike shop that the 19" tires are all lower profile, making the effective tire size roughly the same.
He said that the reason the off road bikes all come with 18 is the higher (deeper?) sidewall allows for more give which should save you from flats.
'Course, he could be full of it. The whole discussion started with him telling me that that Starcross was a great rear tire, but it only comes in 19, which my bike doesn't have.
Anyway, it may be worth checking into, especially for bcVulcan as it will answer your question.