<p>If given the choice, which path would you take?</p>

<blockquote>
  <p>ASP.NET Webforms + ASP.NET AJAX</p>
</blockquote>

<p><strong>or</strong></p>

<blockquote>
  <p>ASP.NET MVC + JavaScript Framework of your Choice</p>
</blockquote>

<p>Are there any limitations that ASP.NET Webforms / ASP.NET AJAX has vis-a-vis MVC? </p>

I love webforms, but ASP.NET AJAX is a pile of crap.
I prefer to use WebForms + custom HTTPHandlers handling the server side of any AJAX calls.
Heh, downvoted...
ASP.NET AJAX Is a pile of crap because a callback requires the entire page class to be reinstantiated, you aren't calling a single method, you are rebuilding the entire page on the server everytime.
Also, UpdatePanels return the entire page, only the section in the update panel is popped in, its a total waste of bandwidth.
I understand why its done this way, because WebForms controls can't really be easily other ways, but it still is really lousy.