At the moment Musashi would be the better choice. With Kiril you have already a healer, that boosts you defense like Vivi (and he also boosts your attack). So a fast attacker like Musashi would be great to do high damage.
Of course Vivi heals more than Kiril, but then you would need another blue one (if you didn’t want to use two healers). Furthermore you would barely have a strong attacker.
I’d do Viv if you’re taking a long view - she’s only of only a few heroes that has cleanse (takes debuffs and DOT off your team) and her massive defensive buff is devastating for the other team if not dispelled. That’s on top of the heal. Plus a 714 attack stat is nothing to sneeze at.
Musa’s good, no doubt, but there are many other fast attacking 5* heroes that are better than Musa (I’d run Joon and Viv attacking on raids before doing anything with Musa. Same for maps/quests).
It’s a win/win, but I would choose Viv as it’s easier to get fast attackers elsewhere than it is to get what Viv brings to the table
It also depends what else you have on the team. Ares and Alberich are popular. With those two, adding Vivica is healing overload IMO. With his self-heal, Musashi is an obvious choice to live outside of Ares’s HP regen. Something like:
Musa isn’t scary without Ares buffing him on defense.
I ascended Musa personally and don’t have Joon, but Vivi is still level 1… as someone correctly stated Alby is currently in all my attacking lineups and carrying two slow heroes on attack is begging for board trouble regardless of what fancy mana troops people have.
Viv on defense is a bit meh; that said, Alliance wars or new titans might push Vivica higher than Musa as Musa really doesn’t bring anything unique to the party… Joon > Musa and I don’t think anyone really would argue that one, but as stated I don’t have Joon.
I personally leveled Musashi and like him a lot. He is fast and a hard hitter and I kind of like the self healing too😊
For my team Viv would be too much healing and you may have better use for Musashis skills now, considering the heroes you have. It depends on your priorities both now and in the long run.