Like Phillip, I am also thinking about building a UV box. In fact, I have a "design" sketched out and starting picking up materials last week.
Linus is a ULF & Pt/Pd printer who posted in another thread a couple of months back that he uses the 20w. spirals in his UV box. I saw some of his work at the LF conference last year and it was gorgeous. I suspect that there was more involved in achieving that look than just his choice of bulbs.
In my case, I am thinking about a box that will allow me to print up to 10x15". It will have six bulbs on 5" centers. I'm thinking about using the spiral bulbs because my experience is that spiral CFL bulbs, with their integrated electronic ballasts, tend to give better starting performance than inexpensive tublular fluorescents with magnetic ballasts. My initial run at a cost estimate is that a box using spirals will run about the same as a box using cheap tubular fixtures - to get the better electronic ballasts with tubular fixtures will cause that option to be much more expensive than a design based on spirals.
One other thought - with tubular fixtures, the recommendation is to line the top of the box with a sheet of metal. This supposedly helps assure that all of the tubes start when you turn it on, and it also provides a reflective surface. I don't think that the metal sheet is required with spirals, but I'm still planning to use it for its reflective value.