One item you may have overlooked - its important to remember that the "user experience" in all of this is poorly served in both areanas of dbms's. For example the pivot control is hardly useful for any serious BI and the oracle express tools (name has probably changed) are clunky and somewhat limiting. When making a decision, take a careful look at the packages to deliver the real functionality to the end user such as Cognos and others (I cant remember the freaking names!). These tools all "plug in" to the underlying BI infrastructure and their associated API's, the plug in's may also be limitinng based on the company/version your connecting to, and worst still, if the GUI vendor does or does not support XYZ feature from the dbms vendor's cubes/bi framework. The other end of the stick is that the GUI vendor like cognos also gens its own the cubes, with a good security model wrappered around it; so then the matrix of choice broadens for you...
Dont expect to be coding up screens using API's, it may be very costly for you, and there are a huge number of vendors out there will all the hooks to MS and Oracle with extremely function UI's for your hardcore and "standard" users. Perhaps code up the "executives" BI screens yourself, but Id be supprised that you can pull off the rest.
Author of "SQL Server Backup, Recovery & Troubleshooting"
Author of "SQL Server 2k for the Oracle DBA"