[silentbicycle]

I've had and enjoyed both. The latter has less about the fundamentals, but adds chapters on real-time GCs, concurrent GCs, and other advanced material. (Most of whose research probably came out after the first edition.)

If you're interested in GCs, I would suggest getting the newer edition for advanced material, and deferring to freely available papers such as Wilson's "Uniprocessor Garbage Collection Techniques" for fundamentals.