Between R12 and 11i, which is the preferred technology to learn. For someone who wants to work in a big 4 Accounting firm such as Deloitte and Touche or PriceWaterHouseCoopers, as a junior auditor or who aspires to be a CFO in a large multinational firm.
There are many technologies available in 11i/R12 and it depends on the role you are planning to do.
As a DBA, you need to be aware of the database administration, Oracle AS (9i/10g) administration, Oracle Developer 6i/10g administration, ..etc
As a Developer, you need to learn some (or maybe all) of Developer 6i/10g, XML, Java, ..etc.
As a functional consultant, depends on what modules you are planning to maintain (Financial, HR, CRM, ..etc).
Applications Releases 11i and 12
Oracle E-Business Suite
You may also search old threads for similar discussion, it should be helpful.