Mark - there is a wide range of issues to consider for LF studio portraiture. Which equipment combo is right for you depends, I think, on how you see subjects (your portraiture style) and what size and type of prints you want to produce. Oh, and budget may play a role, too.
If you need to make enlargements of various sizes, 4x5 is probably the most practical film format. 4x5 enlargers are relatively inexpensive, and more plentiful than 5x7 enlargers on the used market. Going 8x10 usually means making contact prints only - unless you have a big budget combined with a big darkroom.
As "jdef" noted, the choices also translate to studio space requirements - particularly if you factor in your portraiture style. For example, my style preference leans toward tighter, head-and-shoulders images. But, I also prefer the perspective provided by greater lens-to-subject distances. That, in turn, translates to using longer focal lengths to fill the frame at the more pleasing lens-to-subject distance, and the longer bellows extension needed for the longer lenses. Thus, I personally lean toward using a lens that is a bit longer than 2x the "normal" focal length on 4x5.
With the limited bellows extension of a 4x5 field camera (a Toyo 45AX in my case), I could get by with a Nikkor 360T (telephoto design) lens within the space I had in the studio at my previous house, which allowed me about 12' or so between camera and subject. (Telephoto designs require less bellows extension than conventional lenses.)
The "classic" LF portrait lenses (somewhat soft and "glowy"), however, tend to be only slightly long for the format size for which they were designed. Thus, I lean toward using more modern lenses that fit the physical constraints of my gear, and then use filtration to achieve the softer image when I want it.
After I added an 8x10 field camera to my kit, another option is now available to me. I can shoot 8x10 using a lens closer to "normal" focal length, or I can use a 4x5 reducing back on the 8x10, and take advantage of the longer bellows on the 8x10. (I bought a Tachihara double-extension wood field camera new, as it was the most economical new camera quickly available.) Interestingly, the same Nikkor 360T, while designed to cover 4x5, will actually cover the 8x10 at close distances with my Tachi's bellows racked out almost all the way. At that distance, however, the size of the camera starts getting "intimidating" for the subject.
Bottom line, my suggestion is to think through all of these considerations, playing out the practical considerations in your mind, and then arrive at the solution that is optimal for what you want to do, portraiture-wise, and how that fits with other work you want to do with LF.