I think you should avoid ExtJS. I believe you should avoid it because it's considerably heavier, hard to look at, and most importantly, there are some other people where I work that use it, and I don't envy them at all.
I feel that their development is held back in large part by difficulty understanding ExtJS, and difficulty finding documentation for a lot of things. Don't get me wrong, ExtJS has a very extensive man pages, but a lot of it is just descriptions (sometimes incomplete) of classes and methods -- where as when I last checked, the official guide was lacking.
I'm about 90% sure that ExtJS can dynamically load modules. It really depends on what kind of performance you are looking for for your app. There are some benchmarks out there, this one should help:
http://jsperf.com/angular-vs-knockout-vs-extjs-vs-backbone
Like acesubido said however, Sencha is a resource that the devs are free to tap into and get support from if they buy a Sencha contract. This will be extremely important if you feel the team will have trouble adapting.
However, my general opinion (as stated above) is that ExtJS is more trouble than it is worth. From what I have seen, my colleagues spent more time making ExtJS components and trying to figure out what went where than building useful functionality. Also, from what I have seen (though this may be specific to my witnessed case) - choosing ExtJS means basically moving away from markup, in general. When looking through the code base of my colleagues (I had to help them debug their app alot), there was a metric ton of JS, but almost no HTML. ExtJS, though very powerful, abstracts almost too much from markup, and gets very heavy very quick.
If you're wanting them to learn HTML5 as they go, or even any HTML at all, they're gonna want to at least get a chance of writing it.
Please take all this with a grain of salt, I'm very biased against ExtJS because of how much trouble I've had with it -- as a confident developer I'd choose Backbone/Ember/Angular... If I wanted the safety net, I'd choose ExtJS