Web dynpro model: Class or BAPI o WebService ,What is the better method to use in each case ?

Hi All,

My doubt is What s the method that is better in these case ?

1) A small web dynpro application.

2) A big application with several views dependent and share data...

3) A reusable application for other apllication.

And, What is the advantage of a Web Service ?

Thanks

Diego.