I use the plastic well one the most, as it is small; that works well for my space. I usually paint on canvas sizes that are around 8-1/2 x 11 inches, so it also holds pretty much the amount of paint that I need.
The other thing I like about the plastic wells, is that once I'm finished painting, I set the palette aside and let the paint air dry. (I actually have several of these in case I'm working on multiple projects and/or at different times.) Once the paint is dry I peel the dried paint out of the wells. Most of the paint peels off pretty easily, and the rest I do a few scrapings with my finger nails, or quickly wash it out with soap and water. That might seem a little time consuming; however, I'm keeping less paint from being washed down the drain. Then it's only what is coming off of my brushes that goes down the drain, so I see that as an environmentally friendly thing to do.
Cheryl Paton