Ask HN: Flex or JavaScript. Which way to go?

I am adding features to a large web based enterprise application shared by several companies. The sever is Jboss/Java/Spring/Hibernate. The client side is HTML/JavaScript with a small amount of AJAX. 400K lines total. The appearance is ugly as sin and performance is slowed by too much page loading and 200k of JavaScript libraries( fixable).

5-10% of the application has been converted to Flex and the client loves it. The interface is stylish, clean, and responsive. The client wants the new part done in Flex. Nothing in the Application uses any advanced features like video or animation.

My objections to Flex are several. The full conversion will take a year or two, because it needs to be rewritten. For that period there will be a Flex interface and a JS/HTML interface, right now in different windows. I think this will be awkward for the other companies participating. The HTML interface can be cleaned up using a little design and CSS and rewritten as needed.

In any event the client side needs to be rewritten.

I think, but have not verified, that there are a lot more third party JS widgets than Flex.

Flex and JS can talk to each other, but they use different DOM implementations. I wonder how easy it will be to keep a client side global state while loading different sub-pages.

And I dont know Flex. This is not insurmountable, but it will slow me down.

Thoughts?