I would say it depends on what you were shooting for the most part. 500 will excel in lower light situations. More lens options available for the 500 (if you need to rent, wide or stronger tele). So questions like which is better with zero info can be misleading.
So it is not a good vs bad choice it is which one fits your needs better.
It is that CMOS chip in the 300.. If you are shooting all the time in a controlled situation than get the new camera and 5 year warranty. But I shoot everywhere, with and with out my lights so I got the 500 and I am looking for another.
FCP, Mac Pro, Mac Book Pro, HPX500, HVX200, Betacam, Dvcam
Write for the Edit, Shoot for the Edit, Edit.....KISS Principle