I was listening to Josh Berkus speak to Laporte and Schwartz about PostgreSQL versus Oracle on FLOSS Weekly, and a real bona fide gem emerged.
Josh relates the difference in product offering between a Sun supported PostgreSQL and a typical Oracle offering, and it isn't the price difference (significant though it may be) which is the real issue, it is the difference in expectation between Sun and Oracle.
Oracle need to sell the database as it is their primary product. At Sun, if the PostgreSQL business breaks even that is well enough since it wasn't the primary business: the platform is. PostgreSQL is just part of a stack making the whole platform look more appealing.
So as a commercial database consumer, you have a choice there, and the choice goes to the heart of vendor relations quality (upon which most other cost and performance factors can be demonstrated to depend). One of the vendors will want you to use the product because it will create a direct revenue stream for them, and the other has no direct commercial interest in you using the system, though they wouldn't mind it if you ended up liking it and looking at their other products.
Of course, this is over-simplifying matters a little. But when you're talking vendor relations management strategy, you're talking long term and when you talk long term, it's the broad brush that paints the clearest target.