Compression lock is probably my favorite lock type after owning a PM2 for a while. I really like the speed of deployment, the ease of closing and the apparent strength. The complaints of how difficult it is to close the liner lock of the GB worries me and is probably the only thing that will prevent me from owning one in the future.
For your first Spydie, I dunno. The PM2 is classic Spyderco. The GB however is one of the best bang for you buck production knives on the market. I would and did go with a PM2, and after having one for a while, would do so again in a heartbeat. If you don't like it, reselling it for at or close to what you paid for it is easy.