Stephen,
The difference between XLS and XLS Extra is the core material. XLS is standard polyester core and the Extra has a Dyneema core. The benefit of the Dyneema core is that it stretches far less and is therefore able to translate puffs into power intead of giving away the extra energy to stretch. Also XLS Extra is stronger and lighter. Switching to Extra would allow you to downsize to 5/16" or even 1/4" and still have a superior line and save you money.