I don't believe MathWorks are planning at all to stop making xUnit available, so you can continue using it if you like. xUnit had not seen any large changes for quite a while in any case, and even though it won't be developed further in terms of features, it may receive an occasional fix if any are needed.
I have tried out the new framework quite a bit, but have not used it on any large projects yet. Previously I have used xUnit on large projects. However, I'm no expert on unit testing - so please read the following opinions in that context.
I'm pretty sure there's nothing you can do in xUnit that you can't do in the new framework. In general it's much more flexible and powerful than xUnit, providing additional features and a better way to organise and structure your tests. It's a lot easier to set up and tear down suites of tests, to manage and close resources (files, figure windows, database connections etc), and to carry out tricky tests such as checking that the right number of arguments are returned.
However, whereas a typical xUnit test was implemented as a fairly simple MATLAB function, tests in the new framework are typically implemented (in 13a, but see below for 13b) as classes using MATLAB's OO syntax, and if you're not comfortable with that it may seem like a big leap.
I should also add that although the documentation for the testing framework is excellent as reference material, I haven't found it to be great as a tutorial.
In 13b, the need to use classes has been offset a bit with the introduction of the functiontests command, which creates a test suite for you from a file containing tests implemented as local functions. That will make things much easier if you're not comfortable with class syntax. But I would think that if you want to take advantage of everything, you'd probably still want to use the main framework.
Hope my experience is of help - if you're lucky, perhaps @AndyCampbell will chime in...