Offhand, the OPC seems to run a tighter ship doctrinally and liturgically than does the PCA. There's a certain strictness, if not rigidity in some respects (culturally, for instance).
The PCA definitely seems to be more 'open' to the secular culture, reaching out to the urban culture, young professionals, university students, etc., from all walks of life -- and who may or may not be, as of yet, orthodox or even close to it. (There's a Tim Keller-esque, liturgically 'free-form' PCA church a few blocks away from my campus which is growing in popularity among the evangelical students, including Arminians.)