The ride quality difference going from 19s to 20s (or from 18s to 19s, 20s to 21s etc) is dependent on the road surface. On a perfectly smooth road there's no difference at all but the worse the road the more you'll feel the difference.
Unless you're a really keen on the look of the 20s, I stick with the 19s. You'll find also that 20s will be generally more expensive than 19s within any particular brand. If you do a search through the various threads on tyres you'll also find that people with larger diameter wheels and wider tyres complain more about uneven wear than those with smaller diameter wheels and narrower tyres.
I find it interesting that the BMW X3CCs than run in the Paris-Dakar – that are obviously designed to go very hard on all sorts of crap surfaces – run very narrow but tall tyres (235/85) on 16-inch rims.