Now, we don't know exactly when 6th edition will be available, so if you want to start a game now, go ahead with 5.2. Many of the rules will be essentially the same, you will get experience with the system, and depending on your desire, 5.2 starts with Uther (Arthur's father), so you can run a campaign of 25 years or so and then update to 6th edition when it comes out.
Morien, on the KAP discord, has excellent suggestions on how to do this.