Speed!!!
I did some real estate sites and I really really regret having to use Access!!! I would have rather gone with MyODBC.
Mysql is MUCH more stable, quicker and easier to use.
One thing that I could not figure out in Access and PHP was how to use a Next/Prev function. PHP / mysql work well on that particular functionality.
But if you want to see how slow' access is, goto http://www.godfreyrealty.com and you will see what I mean. Click on the Residential listings! VERY SLOW... But the client insisted on MS!
As far as Oracle, I've never used it. I'm sure its very reliable. But you must have heard how Nasa has gone to Mysql over oracle.
Hope that helps.