Scuttles really are a product of what you are looking for in one. For those that like mixing suds in them, you need a wider bowl and that gives a way some of the heat preserving, but a little water in the bottom of the top bowl helps. When you are just looking for keeping the suds warm thought the shave, then a 3 inch or so wide bowl by 2 1/2 inches deep is good. The Moss scuttle was really the first that had enough water between the two bowls to really maintain the heat for the brush though multiple passes. When I was looking to improve on the Moss, I went with the pitcher looking type scuttle which you see others doing these days. I find that these really are the best for keeping the suds warm through 2 or 3 passes of shaving. Many of the bowls are good for lathering. I am still a fan of face lathering and tend to let my brush soak in a filled upper bowl of a scuttle prior to putting the first lather.
Have fun.