In the previous public betas, the code sent to the PB is almost the same as the code sent to the Developer Preview (DP). The only difference I can remember is that the PB had a Feedback Assistant, and you could log in with the Apple ID on the Public Beta and send feedback. Developers on the DP needed to log into radar.
Since the code is the same, the biggest difference will be the frequency of updates. DPs are updated more frequently than betas. I think there were usually two DPs per beta, but I didn't really check. The advantage of the DP is that you can track more closely what is broken and what gets fixed: you can recheck the bugs you've reported and see if they have been fixed. Or if you need to find a workaround. The advantage of the beta is that stuff is not changing out from under you as often. You don't need to worry about things being broken in a beta as frequently as in a DP. If it's broken, you find a workaround and wait till the next beta, and you don't have to find workarounds as often as the DP, since it isn't released as much.