How secure is it to receive an offer of post-doc position in email? I received an unofficial offer for a post-doctoral researcher position a few days ago from a well-known and respected professor in the field, and the institution is very well-ranked. At the same time, I already have an official offer from a different place, but not comparable in value. So, I need to know how certain the unofficial offer is to justify declining the official one. 

Actually, after I received the first offer officially, I contacted the other places to which I had applied, and asked them to decide on my application, and set a deadline for them by when I had to decide about my already-issued offer. I received the second offer right before the deadline as an email in clear and strong words ('I am pleased to offer you ...') similar to what you see in an official offer letter. I quickly accepted the offer. 

I think that since he knew I would have to decide about the other offer, he tried to be clear in the email, encouraging me to decline the other one. However, I'm not sure whether it would be wise to do so. More specifically, does such an email work as a legally binding document (e.g., capable to be held up at a court)? I'd say the difference between the values of the positions actually justifies 1%  risk. What do you suggest?
An unofficial offer is typically a notification of intent to make a formal offer, and I believe not legally binding.  This is not to say that an offer in email is not official and cannot be legally binding---but according to you the offer is unofficial.  Even if it is legally binding, having to take a university to court to secure a position does not sound like a productive way to start a postdoc.

Assuming the postdoc is completely funded from a grant on which this professor is the PI, in all likelihood the only way this would not turn into a formal offer is if the professor changes his mind or is somehow incapacitated.  Also, formality standards of offers can vary.  Anyway, if you are worried about it, I suggest you ask about the terms (if you have not received them) and when you can expect a formal offer.