Most of the reponses suggest taking offer B with informng Company A ASAP. I faced with a similar situation and joined company A. I feel what ever decision you make you should be comfortable with that. In my situation A had already selected me for a particular project and it would been very difficult for them to arrange for a alternate in a short span of time. I don't know what I lost by not joining B, but I have gained a lot by joining A.
