ALM 12.21 is the current latest version that has a "Web UI".
The Web UI is a genuine HTML5 interface with ZERO client-side components -- all you need is an HTML5-compliant browser (IE10,11, Chrome, Firefox, Safari...).
The upside is that it's much easier to get your dev and BA teams on board with ALM, since there's no client to install. 'Author Mode' for requirements is very cool. I like it a lot.
The downside is that it only contains REQ and DEFECT modules though, so your testers will still need to use the 'classic client' for most of their work.
ALM 12.50 is the latest version of ALM.
It does not have a 'Web UI'.
It's a logical step from ALM 11, and along with the features that came in to ALM11.5x and ALM 12.x (there's a big change in features compared to ALM 11) you will also get
- better integration with external tests: Synch, View and Trace results from industry standard test frameworks such as TestNG, Junit, Nunit;
- Better (proper!) integration with Jenkins
- Global search
- better snapshot tool