The difference in aspect ratio is only "unnoticeable" if you're one of those people who watches 4:3 TV stretched to fill a 16:9 screen with nothing more than a passing thought of "huh, those guys look a little fat for basket ball players". It's not nearly as evident as 16:9 vs. 4:3, but it's still apparent.
However, it may be helpful to you if you want to do both horizontal and vertical without rotating the monitor. You'll need black bars in either mode to maintain proper presentation aspect ratio, but the bars will be tiny in one orientation and smaller than if you're using a 4:3 when in the other orientation.
Aside from the obvious size difference and resolution difference, consider that most of the 5:4 19" panels are PVA or MVA, while most of the 20" 4:3 panels at 1600x1200 are IPS. The latter will generally have better viewing angle, especially if you are wanting to mount it rotated from its normal orientation. This would generally make the 20" more valuable, size difference aside. Confirm before purchase of course; you don't want to get stuck paying top dollar IPS prices for a POS early-run TN panel.