I havent tried to use Google App engine for running Unity, but when you use the Webplayer, the settings you mention (as far as I know) has the following meaning:

1) Streamed, means that the first scene (can be configured) will be show before the whole file is downloaded to the webplayer. This can speed up some of the feeling + later scenes/levels can load in the back while the game starts up faster.

2) Offline, generates/copies the UnityObject.js file into the same folder as where you publish instead of linking to the latest online version at Unity servers.

Look for the details here. Google is your friend. I used: "unity3d web publish" to find this link. http://unity3d.com/support/documentation/Manual/Publishing%20Builds.html