Hi Walter and Walter
Full disclosure: I work for Kendo UI. This means that I'm partial and I know quite a bit more about Kendo UI than I do Sencha.
That being said, let me tell you what I do know about Kendo UI and where it really shines. Hopefully this will help you make your decision in picking the product that is right for your project.
Kendo UI is a complete package. It's everything you need for rich HTML5 web, mobile and data visualization.
Kendo UI is heavily invested in HTML5. This means that we do feature detection under the covers so that you always get the best that your current browser can offer you while still providing a beautiful UI for users on older browsers. For instance, Kendo UI will check for touch events and click events, it will use SVG where it can and fall back to VML in older version of IE, and it will always animate with CSS3 if available.
Kendo UI is built on jQuery. If you are more familiar with the jQuery $, then Kendo UI is going to be very comfortable for you.
Kendo UI mobile has adaptive rendering which will emulate the UI on devices without you having to change anything. This means you can truly write your mobile app once and have it look native everywhere.
Kendo UI is engineered to be lightweight and extremely fast. There has always been a focus on speed from day 1, all the way from the UI widgets, down to the core of the framework where the templates and MVVM framework live.
Kendo UI Iterates very quickly. We have 3 major releases a year and a service pack in between each one. This means more features for you, more improvements on existing features, and the ability to address concerns from our user base very quickly.
You should choose the platform that's right for your project. We believe that Kendo UI is the best HTML5 platform for web, mobile and rich charting, but we are not as objective given the fact that we created it :). Good luck and please let us know if there is anything that you find missing from Kendo UI that you would like to see by visiting our User Voice site.