Edo Itachi. Don't even stop to think about it. Just go for edo itachi straight away.

Okay let's look at it this way: With 6p naruto and sus kakashi lineup, you'll be basically unstoppable given that you are faster. BUT, susanoo kakashi is a one trick ninja. His only redeeming quality is his passive; which to me is kind of not worth his price. Edo itachi, on the other hand, is an overall great support ninja with both c.c. and anti-c.c. qualities, ability to cancel buffs, and relatively safe from being chased as well as allowing you to survive most round 1 blitzes. I may seem biased but I would highly recommend you to get edo itachi over susanoo kakashi. You can still stomp people with 6p naruto even without susanoo kakashi.

But well in the end it's still up to you. Consider your options carefully

edo itachi does all those things and provides more support and auxiliary. Susanoo kakashi's mystery is good but it is described as and meant as an offensive skill but acts and feels more like a support skill since the damage is almost non-existent. Don't get me wrong, susanoo kakashi is good but I would really recommend just waiting for his price to become lower or for an event that offers him for a much lower price since there are other ninjas that outweigh him in terms of value and use.