Rainbow 300s are 16ms +- from the exact note timing on all ODs. It's the only timing window that does not change based on OD.
In standard mania scoring, both 300s are worth 100% accuracy, but a rainbow 300's base score per note (before all the other math happens) is 320, while a regular 300 is.... 300.
In scorev2, only the rainbow 300 is worth 100% accuracy. I can't recall what a regular 300 is worth off the top of my head though. So in scorev2, you can only get 100% accuracy if you get all rainbow 300s (however, I believe the timing window for rainbow 300s is larger in scorev2).