The ones with the plastic boots snap onto the screw on adapters that came with your plugs, the metal ones don't use those adapters and clip onto the small threaded end of the plug, at least that is how mine work, they are the blue iPD "performance" wires with the metal boots. No performance here, I just liked the color : )

As far as my preference goes, I used to have problems with the plastic booted bosch wires somehow shimmying off the plugs and causing the spark to arch to the block. I feel that the metal boots stay on much more securely. Don't forget to used dielectric grease either way!

Justin