This is going to be a horrible meta-post, you know the kind that contains little to no original material and points you to other stuff (like your last post I hear you say).
I like the idea of a post over on the MCS UK blog on SharePoint 2013 Development (Apps versus Solutions) as it concisely runs through the thought process when considering how to construct your solution in SharePoint 2013. My caveat is the inclusion of Sandboxed solutions. Sandbox solutions will not be available in the next version of SharePoint, and if you agree with what is said in Sandboxed Solutions are deprecated in SharePoint 2013 they are only included in 2013 as a favour for those upgrades from 2010 (yes – the version that introduced Sandboxed Solutions).
I guess consideration of application longevity is a bit of a moot point in our industry but I’d be inclined to move Sandboxed Solutions down the list as a bit of a dead end.