I actually use both.
The Bosun's Chair (depending on the design) tends to be more comfortable, like a chair; they often have side pockets for tools, which is very useful.
I also use a harness, and go up with 2 halyards. Yeah, probably overkill, and needs a couple of helpers instead of just one...but the Admiral worries.
The rock climbing harness is generally a simple affair which prevents you from plunging to your doom - not designed for long periods sitting there while you are working. 'Dog summarised it well. Some designs are more comfortable than others, but even so, if you are sitting and working on something, a Bosun's Chair is generally the way to go.
Hope this helps!