If you are looking at the transition from portraiture on 120 roll film (in, I presume, a reflex or rangefinder camera) to portraiture on either 4x5 or 8x10, then the relative merits of the two formats have been pretty well described: cost, bulk and depth of field.

However, there is a big difference between a situation where you see the subject through the camera right up to the moment of exposure, and one where, having composed and focused and directed the subject from behind the camera, you actually make the exposure after a brief interlude of mechanical manipulation, while looking at the subject from beside the camera. Not only do you have to visualize what the camera is actually seeing, but you also have to deal with the subject's tendency to shift attention to you rather than to the camera during this interlude. Or learn to make portraits with the subject looking off to one side

A good place from which to explore the transition might be an inexpensive monorail 4x5 (Calumet CC400 or comparable) with a few holders and a roll film back. A "normal" lens for 4x5 makes a good "longish" focal length for 6x7 or 6x9 portraits, and you could probably use your current tripod, enlarger, etc.

When you find that you really, really want larger negatives, then moving up to 8x10 will be (relatively) painless and inexpensive---unless your interest is in silver-gelatin prints larger than 8x10!