WPF is the way to go, without a doubt. I agree with all that @Ray Burns has said.
Because:
- You will get a richer, slicker, faster application.
- It will be easier to build1.
- Softphone/Scanner (i.e. hardware) integration is going to require browser plugins etc. and this can be a nightmare with a browser based application.
- Keyboard navigation is still better with native applications.
- IME Maintenance is easier with WPF applications.
Definitely use WCF to provide the backend via The Entity Framework, see The Entity Framework In Layered Architectures. You can do have a better integration with the backend in a native application because it can be called inline - no need for callbacks or ajax. I've built components for WPF that are linked via EF to the business logic to provide aware controls for simple stuff like validation. It's stunningly good to drop a customer name field onto a form and it just works.
To add additional components you need to build it with a proper well thought out plugin architecture. This is the same in both environments. I've got some thoughts on this I jotted down in my journal entitled Designing a plugin architecture for an application
When building a WPF application you will be writing in one language (e.g. C#) + markup (XAML). When building asp.net you endup with two languages + markup, as you always have to code some Javascript.
So, based on your requirements it has be to WPF / WCF (EF). A web based application will be a lot more work, more complexity, and not be as nice.
About 12 months ago I was fortunate enough to be given a free hand to choose the technology for a new application. I spent almost a month evaluating all of the options and came to the conclusion that it had to be C#, WPF, Entity Framework. After writing the application I can confirm that it was the right choice...
1. It will still be easier even if your programmers have to learn WPF first. WPF is much better thought out, great and lovely. very lovely. It just works right.