As far as processing there's no difference anymore in IIS 7. If you are running Integrated Pipeline mode all the calls should be running through the .Net engine. But some methods such as SQL session only works in ASP.Net. ASP sessions cannot be stored off the application pool. That's one consideration. And elements such as system.net.mail can be set on the applications web.config file, but only ASP.Net (.aspx) pages will access that. In ASP, you will need to fully code the email handling for each page.
