Its a bit like asking "what is the best way to choose a supermarket to shop at".
@lachat, you talk about long distance driving to Spain, wild camping, CQC and survival. Is this use of a bivvy initially going to be recreational, somewhere in wilder country, or are you looking at how to sleep near your vehicle on a road trip? It makes a big difference in what people might advise when choosing a spot. More remote spots will generally be safer of course, so things like damp ground, cold sinks, animal trails and a nice view might rise up the list above security, concealment, escape routes, or checking the ground for needles.