Mark, forgive me if I've underestimated the extent of your tool addiction.
Before somebody places an order, keep in mind that Doug Thompson offers 4 varieties of spindle gouge (if I remember correctly). One is a fluteless gouge and probably not what you want. One is just called "spindle gouge", and 2 say "detail gouge". One of the detail gouges has a pretty shallow flute (detail gouge) and the other has an even shallower flute (shallow detail gouge). All of them are strong and do a good job, but it can be a little confusing to get the right one until you've tried them out.
In my search for that one tool keeping me from greatness, I've used them all and here humbly is my experience. The "shallow detail gouge" is fairly specialized, and handles much like a fluteless gouge. Either of the other two gouges work well for me and can do pretty much any task I want. The regular spindle gouge is an effective, all-purpose gouge, and if ground to an acute angle (25-35 degrees, say) with the heel relieved can do a reasonable job on many of the tasks for which folks get a detail gouge. If you're going to make lots of finials, or back hollow lots of boxes, the detail gouge has an advantage.
And I vote for 3/8", too.