Hexblade Fighter Multiclass level split

Ok, I'm a pretty new player so forgive me if these are dumb questions. I'm building a character that's mainly hexblade warlock with a few levels in fighter, probably battlemaster, using 5.5e rules (yes I know hexblade isn't an official class anymore, just roll with it). My question is about how many fighter levels to take. My DM says we'll get to level 20 this campaign, so I can choose to do either a 17 3 split or a 16 4 split. If I do 16-4 I can take an extra epic boon feat (at least I'm pretty sure, correct me if I'm wrong) and get my charisma score to 22, but if I do 17-3 I get level 9 mystic arcanum. Not sure which one would be better, what does everyone else think?

Hexblade is an official subclass that's perfectly valid for use with the 5.5e rules.

I would ask: what level are you starting at?

If it's less than 16, my advice would be: do not worry about this now at all. Play the character for a while, learn the game more, and see what parts of it you like and what parts you don't, and let that inform your decision much, much later. It's going to be a long time before you need to actually make this decision and there's a good chance you'll change your mind along the way anyway. 

(And yes, the rules as written do allow you to take two Epic Boon feats if you take 16 levels of one class and 4 of another in a very specific order, but it's not clear that this is an intended interaction and some DMs may not allow it.)

Oh so they did add hexblade, I could never get a straight answer out of Google. That is a fair point, I'm only level 7 right now so it's a long ways off, but thank you for the clarifications!

Quote from BurningCar >>

Oh so they did add hexblade, I could never get a straight answer out of Google.

There hasn't been a 5.5e version of Hexblade yet, if that's what you're saying. The 5.5e rules allow using 5e content that hasn't been updated yet.