I use Plex All to protect and clean the headlights, have not used Plexus in a long time but it is a good product too. I highly recommend the Plex All, it has worked really nice, and it better to keep them protected now, then in a couple years trying to fix yellowed lights.
The Diamondite is basically a sealant for clear plastic or glass. With up to 3 months of protection. DP Plex-all cleans and protects, but probally not as long as the diamondite. If you use the DP every week, you will be getting the same protection. Large can of DP plex-all 19oz for $14.99. Diamondite 4oz for $15.99. I would just get the DP I use it a lot, great product.
Another vote for Plex-All...as someone who has a clear bra, I love using it for regular cleaning. The Diamondite Clear Plastic Liquid Armor will have better durability, as it is a dedicated plastic sealant as opposed to Plexus or Plex-All which are "all in one" products. Kind of like comparing the durability of a spray wax to a paste wax.
BTW, Plex-All is $14.99 for 19oz as opposed to $17.99 for 13oz of Plexus and even works a little better in my experience. :xyxthumbs: