If you want to spend absurd amounts of money wrapping boat anchors in rubber, get the 20s. The wheels, HRE's in particular, are significantly heavier at 20", the performance won't be as good, there aren't as many max performance tire options in that size, and the tires you do find will be even more expensive than they already are in 19".
Of course, you could make the same arguments about 19s over 18s, but the difference between 18" and 19" wheels isn't as compelling.
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