I have been asking the question about this subject to some oracle pros in the area. Their reply is that if you are certified for both 8i and 9i, you are more likely to get an offer than simply with a 9i. A lot of companies here are committed to 8i for a long while, the main reason being 9i is too buggy, and the upgrade considered unnecessary for now and a pain in the ass to boot. In fact, a lot of companies still use 7.x, with a few still earlier versions. Also, it's cowtown. Anyone with differing views?
Trying to remain objective, since one can always be wrong.....without knowing it at the moment.