The gate-versus-ball question usually gets answered by whichever is cheaper on the day. It is worth ten minutes of thought instead, because the two valves fail in completely different ways and only one of them can be proved shut without breaking the line.
How each one seals
A wedge gate valve seals by pushing a tapered disc down between two matching body seats. The seal is made by the seating force the stem applies, and on a solid wedge it is made on both faces at once. That means a gate valve can hold in either direction and does not need line pressure to seal, which is why it is still the default on utility, water and general hydrocarbon isolation.
A floating ball valve does the opposite: line pressure pushes the ball into the downstream seat and that is what makes the seal. It is very tight when there is pressure behind it and much less certain during commissioning, hydrotest, or any condition where the differential is low. Trunnion mounted ball valves fix this with spring energised seats, which is one of the reasons they cost more.
Operating time and torque
A ball valve is a quarter turn. On a DN200 line that is a few seconds with a lever or gearbox. A gate valve of the same size is many turns of a handwheel or a bevel gear, and the time to close scales with size. If the valve is part of an emergency shutdown scheme or is cycled more than occasionally, the gate valve is the wrong tool regardless of its price.
Against that, gate valve torque is largely independent of pressure once you are turning, whereas floating ball torque rises with differential pressure. On large high pressure lines this is what pushes the selection to a trunnion design and a bigger actuator.
What happens when they are left part open
Neither valve throttles. A gate valve left half open lets high velocity flow across the bottom edge of the disc and across the seat faces, and the erosion damage is permanent. A ball valve left part open exposes the soft seat directly to the jet, and the groove that cuts into the seat means the valve will never seal again. If a line has to be regulated, the answer is a globe or control valve, not a cheaper isolation valve turned halfway.
Proving the seal
This is the argument that usually settles it. A trunnion mounted ball valve with double block and bleed lets you close the valve, vent the body cavity through the drain, and watch whether either seat leaks. You can prove isolation before anyone opens the line downstream. A gate valve gives you no equivalent test: you close it, you observe whatever the downstream vent tells you, and you rely on the valve having been tested when it was new.
For maintenance isolation ahead of hot work or vessel entry, that difference is worth paying for. For a valve that is opened once at commissioning and closed once at the end of a turnaround, it usually is not.
A short rule of thumb
Frequent operation, emergency shutdown, or isolation that has to be proved: trunnion ball valve. Rare operation, large bore, full bore required for pigging, and a budget that has to stretch across a lot of valves: gate valve. Anything that has to be regulated: neither.