Here's a good discussion of Classical Guitar String Basics (no brand names). Notice he says a set of strings should last about 3 months for a 2-hour-a-day player. During the brief period of time I took beginning classical guitar lessons (early 80s), my teacher recommended Augustine strings. If this isn't your primary instrument, any reputable brand would probably work. The difference in sound could be due to a difference in tension between the old and new set. Good luck on getting your baby back!

Aloha,
Mark