If you have an offer from MRL Boston, take it only if you consider it short term and a simple way to get into industry. Good science goes on, but very poor management. There is also the threat of a massive layoff right down the road (notification end of July?). Attrition is rampant due to lack of respect and viable opportunities (but then again you are in a very active biotech hotbed, so no real hardship there if you hope to stay in Boston).
My advice is take the offer (it should be much more than MGH/HMS--2 to 3x more), but do NOT settle in, do NOT get complacent. Learn as much as you can, network as much as you can, build your resume, then get the hell out of there the VERY FIRST chance you get.
But, with all honesty, I'm really shocked they are offering post doc positons--there is a hiring freeze in all of Merck.
either way, good luck.