Sheet film has all the advantages that sheet film normally has. However, if it were me, I would probably choose roll film. I would also probably get three backs if I was "seriously" shooting this format for a lot of my pictures. I would definitely get at least two.
First of all, there are some emulsions you can get in 120 that you cannot get in sheet sizes. Fuji Reala, Portra 400VC, Fuji Pro 400H, Pan F, Delta 400, Delta 3200, etc. I wouldn't use the Delta 400, but all the others are some of my favorite films. There are also the two 800 color films by Kodak and Fuji. Yes, I would use fast films in a view camera on a tripod, because I like the way they look. It always bugs me that I can't get many of these emulsions in sheet sizes. Second of all, roll film is cheaper per shot, because you are not cropping as much when you print. Third, the bulk and weight issue is a great advantage of roll film. Film holders are the #1 hassle for me when lugging a 4x5 kit around. Then, you have no need for a changing bag if you want to reload. Finally, I think it is wasteful to shoot so much that is just going to get cropped out, unless you want to utilize the advantages of sheet film that I mentioned, such as individual development.