Use separate valves if the tanks aren't vented to the same
source. For certified aircraft, any gravity-feed system that uses one
valve or a valve with a "Both" position must have the tanks vented to
the same source to keep tank pressures equal. Many homebuilts have
problems along this line and a few have quit and crashed because the
builder made mods and/or didn't understand the reasons the designer
made things the way he did. Even some designs had shortcomings to start
with. The original Glastar design, for instance, had a vent tube under
each wingtip to feed each tank separately, and small differences in ram
air pressure would cause one tank to drain before the other. If the
pressure differential is large enough, the tank with lower pressure
will not flow at all and the engine will quit when the other runs dry.
The Cessna 150 used a single valve to control the flow from both
tanks, teed together at the valve inlet, and the tanks were both
connected to the single vent under the left wing. You could still get
uneven flow if you flew with one wing a bit low; remember that the
tanks are well apart and a slight bank will raise one above the other
to cause crossflow. Check valves could stop that, but they'd have to be
installed as low as possible so that the small amount of head pressure
will open them, and their springs would have to be very weak. My old
Auster had such check valves but still used two shutoffs because the
tanks were separately vented at the caps.
If I was to build another airplane I'd have shutoffs right at the
tank outlets; maintenance on the system is a pain if you have to drain
the tanks every time you want to fix something, and it would be nice to
have them there in case of a leak lower down in the system while in
flight.

Dan