Accepted post-doc and have subsequently received offers for full time faculty position - quandary After my PhD (Mechanical engineering) in 2013, I have been employed as a "visiting assistant professor-TERM" for 2013-2014 at the same university (in the USA). In this time I have been applying for several post-doc and faculty positions all around the world. The following fortuitous situation has now developed:


I was interviewed for a post doc position at a famed lab in Europe and received a job offer. This job offer is contingent on me getting security clearance for this lab and getting a long term visa. One of the ground rules laid out was I would not accept other post-doc positions.
Prior to this post-doc interview, I had interviewed for faculty position at US universities. Fortuitously a few weeks after this post-doc job offer, I have been offered full time faculty position at two other universities in the USA.
Now the reason I did apply for post-doc jobs is that they would help me build my network, publish more and help with an eventual faculty position!
I am in some moral quandary now: I know that I have given the post-doc PI my word and I will not renege on it. However, the faculty positions are definitely more lucrative and long term.
I accepted the post-doc job because I was asked to make a decision soon and since I am a foreigner, timing is everything for me and a "job in hand is worth two in the bush" Groans at quotation.


The options (likely and unlikely) that present themselves to me are:


Unlikely: Postpone the faculty positions to Fall of 2015. I don't think these universities would want to do that.
Likely: Angle for better pay/better title at the post-doc jobs.


I am hoping that given the experience in this forum, people could throw some light on this situation.

Edit: Advantages and disadvantages of these positions

Advantages and Disadv. of Postdoc:


(+) Great change of work, reputable lab, exposure to different work culture, deadlines, work pressure, expanding professional network on both sides of atlantic
(-)1-2 years only, relatively poor pay


Adv. and Disadv. of faculty position


(+) Faculty position nuf' said. Much Better pay, "long" term
(-) Will miss out on once in a lifetime post doc at great lab
Maybe I shouldn't start the answer with the judgey part, but if you don't feel like you can accept the faculty positions because of your previous commitment, why the hell didn't you pull your name from consideration the moment you accepted the European job?  On my personal list of academic job hunting sins, not withdrawing your name from a position you've decided you can't accept is much worse than declining a postdoc offer for a TT.  So, there's really no ethical choice at this point, so you may as well do what's best for you long term.

That said, I wouldn't be so quick to dismiss the possibility of starting the TT job with a delay.  This is very common in mathematics (my field); I've done it, and I know dozens of other people who have as well.  I can imagine reasons it would be harder in mechanical engineering, but it's worth a shot.  An important thing to remember as a job candidate is that once you have the offer, the tables are reversed.  They've shown their hand, they've made a time, financial and psychic investment in you, and their other candidates are slowly slipping away as they wait for your answer.  They really want you to say yes, so a concession like letting you take an initial leave is a small one.  After all, they don't even need to pay for it!  Universities are very flexible about letting you do things if they don't have to pay for them.

I mean, really they should want you to do this, since you'll come to them better trained, with more fresh ideas, and a wider perspective.  So sell it on those grounds, and I bet you'll succeed.