Not sure about Jazz Guitar. However, for String Ensembles, I would recommend either Kirk Hunter Strings, Peter Siedlaczek or the Miroslav Vitous String Ensembles. All are of high quality but not cheap either. In regards to choirs, I would recommend Symphony of Voices from Spectrasonics. You can request a demo CD from Ilio/Spectrasonics (
www.ilio.com) that they will send to you via mail. I don't know if there are any demos available showcasing the Miroslav sets. The Peter Siedlaczek sets can be ordered from
www.soundsonline.com and you can also request a demo cd.
HTH
Fernando