There are at least some instruments that are included both in the GPO and the G Jazz&Big Band libraries, such as the Bb Clarinet. Does this mean that it is exactly the same instrument, or is there a classic and a jazz version? I would like to make some pieces with a "jazz style" clarinet ("swing" may be the right term), but I wonder whether this may be done with the GPO clarinets? (As you already may have guessed I own GPO, but not Jazz&Big Band).
The both have Bb Clarinet, flute, and piccolo, etc. -- but they sound like different versions with different samples than those used in GPO.
My personal opinion is that the woodwinds in JABB seem to blend better with the other instuments in that package if you are playing jazz. But there is no reason why you can't mix and match with GPO. I do it all the time. (For example: you can use Bb trumpets in your symphony, or add the Eb clarinet from GPO to your jazz ensemble, or the Bb Clarinet from JABB to the clarinets in GPO to get a slightly different feeling). I feel that, generally, Garritan libraries sound best as ensembles, supporting solo instruments or voices, rather than for exposed solos.
I unhesitatingly encourage you to go ahead with your jazz project using the Clarinet in GPO. Before there was JABB, those of us wanting to work on some jazz pieces were constantly using the GPO instruments.
Now, I am not really sure if there's a difference between the GPO and JABB Bb Clarinet. It seems like the latter could be "reedier," more piercing in its tone, but that could be my imagination. The other JABB instruments which have their counterparts in GPO are definitely different samples--most notably the Trumpets which are brassier, more complex, and have special programming aimed at jazz performance. The Trombone is also definitely brassier. All of the JABB instruments hold up as soloists, while the instruments in GPO are more tame, intended to blend best in ensemble settings.
Getting back to the Clarinet - You'll find the GPO version very musical and flexible. Make good use of your mod wheel (MIDI Controller #1 for volume and timbre variety)--And if you want to get into automating EQ, you can really start getting some natural sounding variety to the sound.
I hope you post your music in our Listening Room when you're ready!