Green card or student visa (J1) for a post-doc in the US?
Get your green card as soon as possible, do not waste your time with J1 visa. As Patricia said, you have to go back to your country where the visa was issued and you can't come back to USA for period of time (there is 2 years rule) stated on your visa. It is very restricted visa. Talk to an immigrant attorney and find the quickest way to get the green card.