One other thing to keep in mind is, what are you going to do w/ the camera? Many MF cameras can be used like a 35mm camera (to a point). You can carry them around w/ you, they don't attract a lot of attention, they're relatively fast to advance to the next frame, etc. None of this applies to 4x5, which is much bigger, much slower, attracts a lot more attention, etc. So you can't get the same shots w/ a 4x5 that you can w/ smaller formats. To me, LF is more about taking a studied and thoughtful approach, and it lends itself to that sort of thing better than 120 for sure.
I don't think anyone has anything to worry about regarding tonality w/ a 120 neg. It will be great if the right film is exposed properly. Remember, a medium format camera has sharper lenses than a 4x5 camera, as a 35mm camera has sharper lenses than a medium format camera. Having a sharp lens will allow you to shoot at wide apertures and get beautiful bokeh, which can compliment tonality. LF lenses are about coverage and the ability to resolve. It's more about the shallow depth of field that LF allows, as well as the ability to use camera movements. You aren't going to get that 'pop' from a 4x5 neg, I don't care how much larger you can print it, but you can get other neat effects, like the very shallow DOF and selective focus in the gorgeous example above this post.
If you want huge prints then 4x5 is better than 120, probably. But you can get very large, sharp prints from a 6x9, or even 6x6 negative. The ability to get a good, large print is interdependent upon having a great lens, great darkroom technique, the right film, a properly aligned enlarger w/ a good lens, the right subject shot in the right light at the correct shutter speed, and more. It's not about any one thing, size of negative included, it's about a succession of things, and any weak link in the chain will sabotage you if you want great large prints. Or, it's harder than it looks. But the right decisions and circumstances have to occur before it's even feasible. If you're doing studio work or that sort of slowly set up shooting, just use an 8x10 camera or larger, contact print the neg and be done w/ it. It's a simpler approach, but doesn't lend itself to every type of shooting.