I would recommend Silverlight.
Enterprise application usually means that there is no problem using plugin based solutions. Additionally, you would save a lot of time and effort making your application cross browser compatible. With regards to security, you can think of silverlight as a shell, the requests in the background is still AJAX. That means it acts basically the same as a normal DHTML page.
DHTML is also client side in a sense, the server generates the UI and data and sends it to the browser where every page is compiled and shown. The user will take +- 5 seconds to view source and potentially get important logic. With silverlight, it will take a lot longer for the user to decompile the xap file, I don't actually know how. What I'm getting at, is that there should NEVER be important logic on the client side, true for both silverlight and DHTML. Developing in silverlight, most of your logic needs to happen on the server and gets sent to the client, the only logic that needs to be in silverlight is the UI code.
Silverlight uses the same security model as ASP namely forms authentication. It works exactly the same. Silverlight is just a View.
There is some other benefits for silverlight. It is mainly record set based, meaning if you edit 30 rows, they all gets sent as one request back to the server, saving requests. And if you think about it, lets say our app is 5mb. Sounds very big, but it gets downloaded once a day for instance, during the day the user will work on this application, but the only communication is simple AJAX requests. No further UI, tables, divs and so on will be requested, actually saving bandwidth and improving performance (only true for an application being used all day).
Lastly, with silverlight you can implement great user experiences. This issue is not trivial at all. A happy user makes a happy developer. The way in which you can show progress or handle errors will make the user more confident about your application, a very big plus! And a great UX obviously cuts down on training costs.
Clearly there is no room for HMTL in the world of enterprise applications anymore. Silverlight does have a big learning curve, and demands a solid architecture. If you want to know more about silverlight for enterprise applications, google Silverlight Ria Services (Ria Services is awesome), also be sure to check out Brad Adams' talk on Building Awesome Silverlight Applications (MIX '09 video session, and code on his site).
I hope I helped...
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