Unitarians believe that their god is one entity.
Trinitarians think that their god is three people. The father, the son and the holy spirit. These three are distinct, yet are one. This is a "sacred mystery", meaning that everyone can hear about this, but it cannot be explained.
Sorry, but if you can't explain your beliefs, why on earth would you think that anyone else would subscribe to them? I guess some people must think, well, this is beyond the capacity of the human mind. But to me, it sounds like a married batchelor.
Unitarians, at least, don't believe the Trinitarian impossibility before breakfast, and bully for them, but there's not a great many of them. Tim Berners-Lee, the inventor of the web, is a Unitarian. See? Not all scientists are atheists.
Unitarians also don't subscribe to the idea of "original sin", wich says that we're all bad people because Adam ate the apple. They don't believe in the inerrancy of the bible (and I don't understand how anyone can, given the obvious mistakes in it).
So, if forced to choose which of the two was most unreasonable, I'd go for Trinitarianism.
If I've misunderstood the tenets of Unitarianism and Trinitarianism, perhaps someone could comment a correction.