19s run a lower profile sidewall, which is meant to cancel out the bigger rim diameter: It shouldn't affect gearing.
I've not done controlled testing, but my general impression is that 19s reward well set-up suspension more, and punish poorly set-up suspension more.
I wouldn't consider running a 19" rear on a pre-78 bike, but if I was building new wheels for an Evo+ race bike, I'd go for the 19 every time.