Hey Dave-
Don't slam me for saying this, but my suggestion would be to try both of them on something you think would sell in your store (or a sign for your wall, or a personal photograph). Compare the quality of the materials used, the packaging, how long it takes for the order to be processed etc.. Your experience might be different than our (or others) but I am a very satisfied customer.
Also, I believe that Colorplak has accounts with most major poster distributors and you can order posters from them at 50% off that will come to you finished (no middleman) - so you won't pay shipping from the poster company or to Colorplak (though I believe they pay shipping from your location to their plant and you pay for the return.) Lots of edge colors and styles too.
One thing I noticed immediately is that Colorplak's colorbox (which is similar to Plak-its flushmount) is a bit thicker and I do not believe Colorplak uses 1/8" masonite for the face. I thibk Colorplak's product is all MDF and the finished box size is deeper.
Colorplak also does a Deco Box with very deep sides. We did the entire offices of Time Warner Cable in them and they looked great and the client was very happy.