Honestly, it's a personal preference. The 1Z0-815 exam is very different from the 1Z0-808 exam, so you will learn something if you take the 1Z0-815 exam. You could also go right into the 1Z0-816 exam if you hold an OCA8 certification title in order to get an OCP11 title, although there are a number of topics you will need to review. For example, it's hard to answer questions about module services or migration if you've skipped learning how to create a module.
Everyone's experience and education level is different, so it comes down to what is the best fit for you. Whatever path you choose, good luck!