At the top, I partially relate to Telerik, as I have used them for a while and have a relationship with them.
I did not use DevExpress Silverlight controls, but I had to use their asp.net controls in a couple of years. Maybe something has changed with them, but from the support, ease of use and perspectives of the samples that are important to me, it’s good that I have nothing to say from experience.
Moving on, you can find code samples for Telerik in the installation folder. Find the demo directory.
Go online and look for help for each product. This is one of the most important things that I find when working with a control set. This is a company that understands that you buy their controls in order to be more productive, and not because you just want to kill time by learning something else.
In addition, you can download a trial version in 30 or 60 days, including support. Try and discover the case of how to do something with each company. Support helps you understand what you need to do, or the problem you're stuck with is extremely important.
Also, with regard to MVVM, I am working on samples. I have compiled a sample template for creating modular applications using MVVM, Prism 4 and using some Telerik controls.
I am approaching an update containing a Grid and using my map control, but that will be a bit before I can pull it out.
http://www.dotnetpatterns.net/content/185-Silverlight-4.0-Prism-4.0-Telerik-Ribbon-Application-Template
Good luck.
PS You mentioned the Charting controls, and I am not familiar with any of them, so again I recommend that you look at the help and samples that each of them provides. A couple of months ago I had to build a demo using some sensors and diagrams, and the help and samples of Telerik help me easily do what I need, without any preliminary skills in managing diagrams or calibrations.