Client-side technology for big web application

I get order to develop web application (as SAAS). Something like ZohoCRM.
It's my first so complicated app.

Key requirements are:

• One page app (no page refresh)
• Easy features adding
• Modularity
• UPD: Real time data exchange: users-server-users (sockets or long-polling)

So I'm struggle to cope with Javascript (jQuery) and Flash (Flex).

Thoughts about Javascript:

Pros: pluginless, supporting by tablets and smartphones browsers, lightweight (loading faster).
Cons: a lot of job on cross-browsing issues, viewable sources.

Thoughts about Flash:

Pros: no need to fudge with cross-browsing, rich components customization.
Cons: require plugin, not clear situation with Apple products.

So which way I have to go?

Thanks in advance.

UPD#1: It will be a web based application for business. First part is basic CRM functionality. Later step-by-step it'll get ERP features.
Main data operations will be with tables and trees with filtering.
Must work just in browsers so far. But I'm sure that later will appear requirements for tablets and smartphones support.

P.S. After thinking and googling I more and more inclining to Javascript stack (jQuery, backbone and underscore).