Difference? Retails comes with accessories like software, an audio cable, and screws. Bulk is just the drive itself (sometimes you might get a software CD too, but that's not gauranteed).
Worth getting? It's a fine drive and will do what it is meant to do. But with DVD-writer prices so low, the price difference between combos and DVD-writers just aren't that big. So combo drives in general are falling in popularity as people opt to spend a few more dollars to get a DVD-writer instead.
Noise? The 6K greatly improves on noise over previous models. But keep in mind that at max speed, you're spinning a disc at 10000 RPM, and there's going to be noise that is unavoidable with ANY high-speed drive.
Firmware? It just means that you can make the drive rip DVDs faster. If you want to use the drive as a DVD-ripper, then this could improve your rip times. And you can make the drive region-free. That's all.