I am from Egypt and I'm planning a vacation to Spain and France. While filling the visa application, should I choose a single or a multiple entry visa?! If i got a single entry Schengen visa, I still can travel from Spain to France and then back to Spain as long as it is during the valid dates of my visa right?!
As long as you stay within the Schengen area, you only need a single entry visa. Travelling between Spain and France is not counted as a new entry.
You should perhaps be aware of a few pitfalls though. Andorra (on the border between Spain and France) is not part of the Schengen area. You are allowed to enter Andorra with a Schengen visa, but traveling back to Spain or France is counted as a new entry into the Schengen area and would require a multiple entry visa.
Edit: Even if immigration controls on the borders between Spain/Andorra or France/Andorra are very relaxed and some resources even indicate that leaving Andorra is not counted as a new entry into the Schengen area, other resources site stories where controls have been made and the rules strictly enforced. Here es even a story, where a holder of a single entry Schengen visa was rejected entry to Andorra on the bus from Barcelona, since leaving the Schengen area into Andorra would prevent him from leaving Andorra again legally.