What nightly is 6.0b1 equivalent to?

I've been trying out 6.0a2, downloading and installing nightly releases every other day. Today I downloaded the 09-03-05 nightly, and also saw there is a 6.0b1 - is there any difference?

I'm wondering if I should install 6.0b1, or keep downloading nightly releases to help out with testing.

BTW, I'm quite impressed with how stable it feels so far. I've found a few issues with new facilities such as new searching options, but that's to be expected at this early stage.