Rei'o for Zangetsu is better if you're under level 100, because of the Level 7 Hogu + 2x Level 100 Ult Sets
But if you're past 100, Joint Task will always be better due to the ability to cater your spending however you please.
Say what you will, but the best investments will usually be spent on stone packs / level 7+ hogus
I don't think a Level 9 Hogu has been introduced to US/EU yet, right?