Our team tried both. Originally we had a real JS guru on the team, and he wrote a ton of code against ExtJS. It was good stuff, but very difficult for anyone else to understand. He ended up leaving the company, and I inherited the front end code base. The project got put on hold for a while, and we decided to take a fresh start. I went with Jquery UI rather than ExtJS, because it just felt too much like you had to drink the kool-aid with ExtJS. Our team was more comfortable with a hybrid approach.
Every form we write is in bare bones partial HTML, and we use some juiced up JQuery UI Dialog code to display the forms as modal pop-ups. Additionally, we use the UI themes and their css classes even for non-JQuery Ui components, that way when we switch themes, all the elements change, not just the JQuery UI ones. There are only a couple of palatable themes available, but on the whole, it's been nice.
Your situation sounds strangely familiar :)... My opinion is that if I am going to design a form that is going to run in a JavaScript app, I should be able to do so in pure JavaScript. For example:
form = new UberUI.form();
form.append( new UberUI.input( {type: "text", name: "login", label: "Login:", validator: myValidatorFunction} ) );
...
form.show();
On a different note, jQueryUI is much more theme'able than ExtJS, which is great.
Every form we write is in bare bones partial HTML, and we use some juiced up JQuery UI Dialog code to display the forms as modal pop-ups. Additionally, we use the UI themes and their css classes even for non-JQuery Ui components, that way when we switch themes, all the elements change, not just the JQuery UI ones. There are only a couple of palatable themes available, but on the whole, it's been nice.