Sebastian,
These are the two basic pros and cons that I find:
Valencia, con: Both 'castellano' and 'valenciano' are spoken, which can lead to a little language confusion probably, at least at the beginning.
Pro: the beach!
Zaragoza, pro: (I was raised there
), pretty good usage of Castilian (without the 'leísmo' and 'leísmo' that is so common in Madrid, for instance)
Con: no beach