Kentico and Umbraco are both great Content Management Systems in their own ways.
One pretty noticeable difference is the price. To build a website based on Kentico, you have the initial cost of at least £3499 for a Kentico licence, whereas Umbraco (being open source) is completely free of charge. So why pay thousands of pounds for a Kentico licence rather than simply use a free CMS?
It really does depend on the particular needs you have for your website whether it is worth the money.
A site built on Umbraco can do anything a site built on Kentico can. The difference is that Kentico comes with a large number of built in components which can reduce the amount of time developers need to spend developing bespoke functionality.
In Umbraco, developers need to create components from scratch (or install third party components). That is not necessarily a bad thing as it can often be more straightforward to build a website component from scratch that does exactly what you want than to spend time trying to get something that is nearly what you need to work.
Both CMSs have excellent APIs that make it easy for developers to build components to use on your website.
Kentico has a wide variety of features built in which are absent from a basic installation of Umbraco, but which developers could add to Kentico (given enough time and money!). Examples are Kentico's excellent E-commerce, on-line marketing, Intranet and community features (such as web forums). The fact that these are built into Kentico is a big advantage.
If such features are needed on an Umbraco site, the functionality would either have to be either created from scratch or a third party component would be required.
Umbraco does have a marketplace with a wide variety of third party packages, but you may find a package which sounds perfect for your needs only to discover that it has not been updated to be compatible with the current version of Umbraco (leading to it either not working at all or unreliability).
If your site is relatively simple though, and you will never actually use all those features built into Kentico, it could be argued that you would be better off going with Umbraco.
Another advantage buying a Kentico licence has over Umbraco is the support from Kentico included in the price, which is excellent. Umbraco technical support contracts can be purchased separately.
In my experience, Kentico will really make the effort to solve any problems you are having with their product. Plus, the fact that you have actually purchased a product from them means that you have a right to complain if there are problems with it...