I would go 5-box. Either 1 pally 4x dks or 5x pallys
Both are strong in Legion mythic dungeons. You wont regret it and you can still play with friends anytime tou want to.

Personally, I am doing a 10-box. I have 10x dks and 5x paladins. My primary team is item level 865 average. Reason i decided to do a 10 box is allows me to attempt raids and trash and more keystone options. Plus i can mix and match my dks and paladins based on dungeon.

Plus, with an all plate team i can boost a fresh plate team very quickly due to loot trading. For example, i can run my item level 780 dk with my 860 in mythics and mythic +2.

Side benefit 10x multi gathering of herbs/ore. See beeqs thread on it. In short, one node can be tapped ten times.