If your're trying to do the big IN YOUR FACE filmscores with the lush syrupy strings then App. Strings would be the better choice. Also, App Strings 1 contains the entire strings sections, Violins, Violas, Celli, and Bassi. So everything is contained in one package.
On the other hand, with Orch. Strings, you have to buy both Orch. Strings 1 and 2. (Orch. Strings 1 is just the violins and violas. Orch. Strings 2: celli and bassi.) So you basically have to buy two products instead of one. And then to upgrade to the full libraries you have to buy two products again instead of one. But the nice thing about this is that you get more articulations with Orch. Strings.
The magic of the VSL strings is the layering. App. Strings or Orch. sTrings sound good on their own but if you layer them with say chamber strings and/or Solo they sound fantastic. Add the possiblilities of VI Pro into the mix and the results are incredible; as close to the real thing as possible with sample libraries. No other library on the market today comes this close.
I repectfully disagree with David about the Legato patches. If you use the Release fader correctly the Legatos sound just as good with either library.