The differences between a '98 and '99 are quite subtle. Many more changes, both good and bad, on the 2000 426.
I would pick a bike based on its condition, not whether it's a 3 year old motorcycle or a 4 year old one.
I have two '98 YZ400F's that I'm interested in selling, in San Diego. I've got pics, if you're interested.
Tony Williams
San Diego