dB Choices - Oracle, DB2 or Something Else?

An Anonymous Coward asks: "I work company where I currently have to implement a new back end database architecture. It needs to be highly scalable, globally accessible, distributed, robust, fault tolerant and secure. There's a feeling amongst management that our current database, PostGres, hasn't got the grunt we need. Everyone seems to think we should go to Oracle or DB2, but I'm not yet convinced. Can anyone suggest what are the pros and cons of Oracle and DB2? Why should we go for one over the other? Is it possible to implement either of these without paying a fortune in maintenance and subscriptions? Which of these requires LESS maintenance given that we are going to be using the Linux on Intel?"