To replace the sink drain (currently a 3/4" gate valve) and the head inlet (a 3/4" ball valve) with new ball valves.. and then fit new something-or-other to the 1-1/2" cockpit drains.
Gate valves fail by remaining stuck in the open or closed position. This is a rare trait for a ball valve. Definitely, select the ball valve.
Yes, ball valves are far superior to gate valves (at least on a boat).
A couple of other reasons to go with ball valves:
You can tell at a glance whether they are open or closed.
You can close them from fully open to fully closed in one quick flick of the wrist.
Gate valves are not allowed in new boat construction here in North America, and haven't been allowed for decades. Insurance companies usually require that any existing gate valves be replaced with proper seacocks/ball valves as a condition of insurability.

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