How can one compare a postdoc position in Europe and USA?
Let me list a few pros of doing postdoc in the US:


In many EU countries a postdoc in the US is considered to be more prestigious than a postdoc in Europe (assuming that the places you do the postdoc in the US and Europe are at roughly the same level), so a postdoc in the US is in general not likely to harm your academic career should you eventually decide to return to Europe
If you give staying in the US a really serious thought, the postdoc in the US allows you to initiate the application for green card; getting the latter will give you unrestricted access to the job market in the US (academic or otherwise), in addition to the EU one, which comes in handy on many occasions.
Given that you have your Ph.D. (and hence the largest part of your academic network) in the US, you have somewhat better odds at succeeding in securing a postdoc in the US (e.g. because the US people you will apply to are in general more likely to know your advisor than people in Europe, and the people in the US will be less hesitant to, say, give a phone call to the authors of your recommendation letters to find out additional details if need be).