In my experience, there isn't much energy savings performance difference between vinyl and wood windows.  Either option will have a much better thermal performance.  The real difference is cost and aesthetics.  Vinyl windows will save money without a doubt, but the trade off is in aesthetics.  Wood windows have a timeless beauty, are available in many colors/species and are typically made from renewable and highly reusable materials (wood and aluminum).  I always recommend wood windows with an aluminum exterior cladding to my clients, if budget allows (such as Marvin, Kolbe & Kolbe, Eagle, Loewen and the like).   Vinyl is a good choice where saving dollars is your highest priority.  The trade off is highly limited color and profile choice (would you like white or beige?).  Manufacturers such as Milgard do offer a few more options than most, but the palette with vinyl windows is still very limited.

My recommendation would be to find time to stop in at a window show room in your town and touch and feel the differences.  Good luck!