Suggestions for UI technology stack for an enterprise app

Suggestions for UI technology stack for an enterprise app

My team is going to build an enterprise application in the
financial/trading realm. Actually, we will rewrite an old application
using new technologies, because old one are too old and it requires much
effort to make some changes to the existing app.
So, my team is quite familiar with Spring and we plan to develop our
backend using it. We haven't come to an agreement about the framework to
use for our frontend. Part of the team is familiar with GWT and someone
with JavaScript.
We want to extensively use the ajax capabilities of the UI framework.
Also, alternate clients may appear in future, so we're think about a REST
API. But also we care about performance.
What are the modern trends for building the UI part of such apps? Will it
be good to use a REST API with Spring MVC? IMHO, GWT is a good choice but
it ties us with only browser clients.
Can you suggest a technology stack for such cases (maybe from your
experience)?