Sanderson Jones on the Sunday Assembly blog has an answer to the question "What's the difference between the Sunday Assembly and the Unitarians?". This is my perspective.
The Sunday Assembly blog says that the difference is that Unitarians are seen as welcoming people of all faiths and asserts that Unitarians see all faiths as equal compared to their view that “there is no God so how should we live now” is what is important. The are prepared to welcome people of all faiths, however, as everyone can benefit from their Assemblies. He concludes however,
"Before I started the Sunday Assembly I did research Unitarianism and many atheists found attending UU congregations difficult as they could occasionally break out into crystal spirituality, conversations of personal experience of God, and other things that humanists find very hard to deal with."
This statement does in some way highlight the distinction between the two movements. Unitarianism is an inclusive faith and we are prepared to describe ourselves as religious liberals. We are religious because we "unite to celebrate and affirm values that embrace and reflect a greater reality than the self." We accept that everyone has a right to seek truth and meaning for themselves. We nurture the spiritual dimension of life drawing upon a range of traditions as reflected in our objective. This means that Unitarian communities will have people with a wide variety of beliefs and you will find people with very different views from yourself. This we see as enriching not threatening. Indeed, as a previous blog of mine "Do Humanists Sing? " highlighted research that found that humanists are not as rational as they would like to think.
Unitarianism is very different across the world. For example the British General Assembly embraces Unitarianism in all its forms as well as Free Christianity; the remnants of a late nineteenth century movement to unite all Christians in a national free Church. The American UUA is a merger of two liberal denominations; the Unitarians and the Universalists and within which religious humanism gained much strength.
More recently Sunday Assemblies have been described as a “fun alternative to the meetings held by Humanist[s] and Unitarians”. “Why on earth aren't people clapping and dancing around and jumping up and down at those gatherings?" Sanderson wondered in an interview with ABC News.
I have attended a gathering of the Sunday Assembly at Conway Hall. It reminded me of the informal Opening Celebrations of the British General Assembly. Most Sunday services in a Unitarian Church or Chapel are more subdued affairs, partly because the form of worship is more traditional (although the content is often very different from a mainstream Christian church) and partly it reflects the spiritual needs of the attendees. At other times, such as a second Sunday evening service or mid-week activities, unitarian practice can take many different forms; celebration , dance, meditation, labyrinth walking, Taize chanting etc. Unlike Sunday Assembly we have not prescribed or franchised models; variety is truly the spice of life.
A Unitarian congregation seeks to meet the spiritual needs of the individual in the context of a loving community. We are there for the sorrows as well as the joys. We have trained leaders - ministerial and lay - to support and guide the community. Nationally and locally we offer a range of personal and communal religious education for adults and children. We similarly work together to promote social justice. We work closely with other progressive faith and non-faith groups and would certainly see Sunday Assembly as allies not competition.