I have done many a search on this issue with mixed results and ensuing ambiguity. The time has come to nail it down for good. If I'm developing the equivalent of a complex ASP.NET application, and I want to run it on top of SharePoint 2010, is it best to develop each page as a web part or an ASPX application page? The pages are many and varied, but each one interacts with a SQL 2008 R2 backend via custom WCF services.
I have yet to see a definitive and authoritative answer to this question. I'm looking for an official guideline from Microsoft or similar. I found this When to use application pages (aspx) versus web parts? on this site, but I'm looking to dig deeper with specific references. Here's a breakdown of what I have found so far:
The following non-official post would clearly favor application pages for my case. However, it was posted back in the MOSS 2007 days, when SharePoint development was more of a challenge. http://grounding.co.za/blogs/brett/archive/2008/07/13/sharepoint-the-role-of-a-web-part-vs-using-application-pages.aspx
Microsoft, in comparing application pages to site pages, states, “An application page is the best type of page to create if you want the page to contain custom code.” http://msdn.microsoft.com/en-us/library/ee231581.aspx
So far, so good. But what threw me off was this other seemingly contradictory link, also from MS: "We currently recommend that third-party developers develop custom Web Parts, which can be added to site pages, to handle their solution's functionality whenever possible, rather than develop custom application pages.” (http://msdn.microsoft.com/en-us/library/gg552610.aspx)
I'm biased toward application pages because that's what our team has been using all along and has had no problems. Everything from the development experience to debugging and deployment support has been top notch. But internal forces are pushing for us to make the switch to web parts, so I'm looking for justification.