QE: MBS vs treasuries

mfleming1983

QE1 purchased MBS, QE2 purchased treasuries, and QE3 is expected to be targeted towards MBS.  What are the motivations and/or advantages to picking one over the other?  
Bernanke seems to be very interested in the makeup of the Fed balance sheet, I’m just not sure I completely understand his logic.
Source:  http://blogs.wsj.com/economics/2009/01/13/bernanke-you-say-quantitative-easing-i-say-credit-easing/

amqata

There’s an added “benefit” of the Fed buying RMBS…there’re still dozens of financial institutions and even countries that need to unload them. The Fed will say it’s about keeping mortgage rates low, but it’s really just a liquidity pump (as all QE measures are).

soddy1979

That’s interesting. I’m surprised that buying MBS lowers mortgage rates moreso than buying treasuries. I would have guessed the opposite, as the treasury curve typically dictates other rates.
Is the RMBS market relatively illiquid? Is it safe to say that the institutions that need QE the most tend to hold a disproportionate amount of RMBS, thereby benefitting the most?