Heres a good question

It all depends on who you love.

but game-play wise, I think gramps has a much more better skill set. Having Battle Continuation EX, a heal defense and attack up, and a manaburst with an added death resist down. You could say he's a bit overloaded for an assassin.

Musashi on the other hand has a "double-hit" niche which helps her unbalanced deck (BBBAQ) so she could still generate stars and np, a 1-turn invincibility with an added star gen, and a mana burst.

Personally, i'm gonna roll for Musashi as I really like her character design.

You need ST Assassin? KH.
You need ST Saber? Musashi.

By the way, both are triple-buster deck with buster NP. MLBs Halloween Princess/Holy Night Supper/Golden Sumo, or at least an Immaginary Number are extremely recomended...unless you have a Waver, otherwise, they wont be charging their NP any soon...

I'm gonna roll for musashi since I think Okita wont be back for quite some time. But I'll also roll for king hassan maybe some ticket and one 10x roll. im gonna go ll out on Musashi though. Assuming I still have quartz after merlin.

King Hassan is better

It's really about who you like more.

And it's a complicated matter to compare two servants that are different classes to each other. I will say this though, King Hassan fills a niche that is very rare among Assassins, which is a staunch damage anchor among Assassin with decent survivability. Due to the nature of Assassin as a class, most servants within that class are either loaded with Q or a mix between A and Q. Sometimes making the damage a little bit... softer, to riders. Mostly relying on crit or NP to deal real damage. But KH brings the raw Busters and Brave and nifty steroid that other assassins can't really fit his shoes to do what he does.

That said, Musashi is not a bad servant at all. She's actually one of my favorite servant both characteristics and gameplay wise. For plus side, Musashi is a better servant to bring when you aren't running class advantages for whatever reason as your damage dealer. Which cannot be said about KH. Saber class is loaded with raw damage dealers and stat-sticks and Busters that a bizarre kit such as BBBQA-B is not as weird.