Dan,
do you plan on using WAS for anything other than OneWorld? If you do then the edition you buy depends on the 'what else'. However, for just OneWorld the advanced version is the best option. (I'm glad you didn't suggest the Single Server Edition).
If you plan on using WAS only for OneWorld then your best option is to purchase the JDEdwards Technology Foundation Update (includes Webspher Portal Server, WAS, DB2 UDB, etc.). This will save you a ton of $$$ on software and on maintenace.
I've done several WAS installs and I've only every used the advanced edition.
Colin