As a long-term participant in Pagan groups (since 1990) and having recently joined a Unitarian church (for those who don't know, there are plenty of Pagans in British Unitarianism), I have been mentally comparing the two. There is no clear "winner" but the comparison is interesting.
Pagans are more focussed on individual friendships; Unitarians are more focused on gathering in community.
Unitarians are better at including everyone in the community, even if they are different from others in some way. (Lesson for Pagans - we need to focus on shared values instead of differing beliefs.)
In a crisis, Pagan friends will rally round, which is great, but if you want a trained full-time minister, with all that that entails, then you're more likely to find one via Unitarianism.
In terms of age and class and education, Unitarians are more diverse than Pagans. This is probably because Paganisms haven't been around so long.
In terms of the values we embrace, Unitarians are much less diverse than Pagans. Even though a Christian Unitarian may differ from a Pagan Unitarian in the mythology they happen to like, their values are remarkably similar.
Unitarians are better at focussing on values and regarding beliefs as less important. Unitarians have more shared values in common, simply because we are very explicit about what those values are. (Hopefully Pax's recent Pagan Values Blogging Month will go some way towards changing that - and I hope it will happen again in 2010).
Both communities are inclusive and welcoming and non-judgmental.
I'd be interested to hear from UU Pagans on this.
Incidentally, while I am still philosophically pagan, I have stopped referring to myself as Pagan (I now call myself Unitarian and Wiccan) because it is no longer clear what "Pagan" actually means (due to things like the reburial issue).