big al the 99 is a much better choice.One because it is newer, two because 98 is the first year the bike came out and they did not know what the people wanted.Its all about the people and they improved the bike to what the people wanted.Personly though i would go with a cr250, much better bike then the yzf will ever be
I have a 99 YZ 400 and I rented a 2000 YZ 426 all day before I bought the 99 YZ 400. I think the YZ 400 is a little more forgiving then the 426. As far as power I added a White Bros E Series Pipe for better spread of power, it added more rev's. I would buy as new a bike as possible so go for the 99 over the 98. I am going to sell mine since I just bought a YZ250F and don't ride the YZ400 anymore. Good luck.
I owned/rode the '98 and '99 YZ400F. If you could ride them blind-folded, I seriously doubt anyone could tell the difference. The '00 is another story...more power, better steering and suspension. All changes you can appreciate by the seat of your pants. I race a '01 YZ250F now, but thats another story...
For my money, I would look for a '426. There have been a few reliability improvement from the '98 to the '99, so the '99 would be my second choice. Never had any problems with any of my YZF's.