Depends on how well the app is designed, and whether or not it actually offers all of the features available on the full site.
The whole reason I got an Android device was so that I could use full blown desktop websites on the go - mainly because the mobile versions, and, as I'm seeing now, Android apps for the sites, usually don't offer all of the features of the full site.
Of course, apps have the advantage of being touchscreen optimized, but once you have an accurate touchscreen (which is, sadly, difficult to find on Android) and a good browser, that advantages starts to shrink rapidly.
Personally, I try to keep as much as I can in the browser. In order for me to install them, apps need to offer functionality that's not implementable on the web...