When to use WEb Dynpro application and Portal application in NW dev studio

I would like to know what is the difference between

webdynpro application and EP Application using PDK.

Are they comaparable ...Which one has an edge over the other specific to any applications.

I want develop an application related to e-commerce using the SAP R3 as backend for an industiry(where in a registered user can place an order and query on the staus of order. All the data will be stored in R3 ).

I am planning to dev this using NW dev studio , but have an appehension about which application is useful for this kind of application either webdynpro or EP application .

can anybody explain which one has edge over the other ( Webdynpro or EP applcations in context with NW Dev studio) and why.

thanks

PK