I did the full suspension swap and drop was 1 1/4 inches on red vert,
if going with 1.5 drop I would stick with 19's if you eventually do 1" drop
you could pull of the 20's but will like roger said have lower profile tires and bit rougher ride.
Unfortunately you have to decide what all it is your going to do, to make it work.
Upside is you don't have to do it all at once. But knowing your end plan, helps in
making choices, regardless of the order you do it.