Okay, let's get this straight from the start: Sega World Soccer '98 isn't worth buying if you already own Sega World Soccer '97. The changes are too minimal to justify getting both games. But, if you didn't have Sega World Soccer '97 to begin with, then Sega World Soccer '98 is the slightly better game.
The options are much the same, but this time it's also possible to play with club teams. Well, only with English, French or Spanish club teams depending on the language you chose. It's rather stupid that it isn't possible to play with Spanish teams if your language is English, but forgivable. The game still isn't officially licensed by FIFA, which means all the names of players of national squads are fake. But, the names of club squad members are all accurate, maybe because the UEFA did give their official credit.
Actually, the availability of club teams is the biggest difference from Sega World Soccer '97. Some things haven't improved however. The AI is still as terrible as in the first game. Goal keepers are a bit better, but still not as good as it should be. It's now possible to control them as player as well, but I wouldn't advise it, since that's nearly impossible. But some of the actions which led Sega World Soccer '97 to goal are now improved and more accurate.
The game feels a bit rushed - the graphics and animations sometimes appear a bit sloppy, and the commentary is very often too late or too soon. Concerning the graphics Sega World Soccer '97 is a bit more polished. But, most importantly, the gameplay has improved. Sega World Soccer '98 plays faster than its predecessor and the control feels even tighter.
If you don't have Sega World Soccer '97 yet, Sega World Soccer '98 is the best soccer game for the Saturn. The improvements over the original are minimal, but it still is an excellent game. Sega World Soccer is definitely an underrated-series that deserves a lot of respect.