Besides the fact it's a later release?
i.e., why does one have "nRF51822" in the name and the other simply "nRF51" ? Is it because the former will only work on a 51822, while the latter will work on a 51422 as well as a 51822?
Will v2.0.0 work on a nRF51 DK (I'm assuming it will, as I believe the nRF51 DK has a 51422?)