Invited to interview for several postdoc positions - how to proceed?
No matter what you do, or how carefully you apply, a better job can always come up later. 

The decision you have to make is not "is this my perfect job?", because unless there's a specific single position that you have your eye on, there's no such thing; it's "is this position good enough that I'd take it if offered?". If it is, then do the interview, and if it goes well (in both directions), choose between the offers that you have available to you at that time.