Silicone caulk for shower and tub?
That gap is to small. Push really hard with the float to completely fill it with grout, giving it one more shot; next time caulk it. Caulked tub surrounds are one of my pet peeves; I'm the one who gets to scrape that mess out.

Whether grout or caulk, I like Some Guy's advice: prestress the tub by filling it with water as you work.