On a Tridion 2011 SP1 system, you have a choice between implementing SiteEdit 2009 SP3 and the more recent "User Interface update for SDL Tridion 2011 SP1" (also known as Experience Manager). What criteria are important in making this choice, and why?
For example:
Both SiteEdit 2009 SP3 and Experience Manager are currently supported products. But it's clear that SDL's focus going forward is to further extend Experience Manager and not SiteEdit 2009 anymore.
In simple scenarios SiteEdit 2009 may be a bit easier to implement, due to the fact that Experience Manager has a bigger impact on the Content Delivery system due to the prerequisites for its Session Preview mechanism. When I install Experience Manager without Session Preview however, I find that it takes me no more time than setting up SiteEdit 2009 - a product that I've installed considerably more often.
But Experience Manager should typically cause fewer integration problems, due to the fact that it doesn't use a server-side proxy. A lot of more advanced authentication scenarios should simply work with Experience Manager, where they've proven challenging with SiteEdit 2009.
I think the above covers points 1 through 4 of your question. I'll leave number 5 to others, although I already mentioned "Session Preview" as one of the big new features in Experience Manager.