Space is the first thing to think about. The 2, 4ft boxes I use have 8 tubes. Just fine for all the work I do. I also have a 2ft box that I use for demos, also with 8 tubes. It's a matter of space. The 2ft box costs as much to build as the 4ft box. Price on the tubes was the same, balast about the same also.
Hope this helps
ps. Oh Sandy, the AP trophy is in Los Angeles