Before buying the Nova Cole Jaws I would recommend that you look at all possible alternatives. I own the Cole Jaws and use them on my Nova Comet II with a G3 chuck. They will allegedly hold up to a 12" diameter bowl, but I've never tried that size and have done mostly smaller diameters.

The Cole Jaws work for a variety of bowl shapes but aren't universal. There are some shapes which really don't get clamped tight in the jaws. I even bought a set of 2 different types of rubber bumpers (I think from Nova also). In the end I haven't found much of a difference between the original bumpers and the others, which are shaped so that it would appear they hold better for some bowls. I don't believe that has been the result.

I recently bought the Longworth/Doughnut chuck combo set from Ron Brown (ronbrownsbest.com). I believe it is much more versatile and safer than the Cole Jaws, but haven't had sufficient time to use it yet. It is an expensive set though.

I think a better solution might be the vacuum chuck cylinders from rubberchucky.com. You don't need a vacuum chuck to use them on the head stock end with support from the tailstock. I realize that you most likely are trying to get full unobstructed access to the bowl bottom. However, with one of the rubber cushioned cylinders and a pointed tailstock, you can turn almost all of the bottom off and cut the rest.

I guess, in the end, what works best for you will likely be driven by exactly what you want as the end result.

I'm not saying the Cole Jaws are not useful. They are. However, there are some bowl shapes they just don't work well for.