When designing a low-wing light aircraft with two integral tanks at the wings and a collector tank in between, what type of valve should be used to allow the pilot to select the integral tank which will feed the collector tank in a reliable way? A selector valve or two solenoid valves?
Fuel will be fed from the integral tanks to the collector tank via jet pumps with no moving parts, not gravity.
There's at least one strong reason to prefer manually operated valves over solenoid valves: they still work during a complete electrical failure.
Aircraft engines, due to magneto ignition, will continue to operate even with (for instance) an exploded battery, melted main ground cable, or other failure that kills the entire electrical system and can't be repaired in flight. You'll have no landing lights, instrument lights, clearance lights, radios, or electical instruments -- but at least you'll have power, basic vacuum or airstream powered instruments (airspeed, altimeter, turn and bank, assuming these analog gauges are installed) and presumably if you have retract gear you have a manual method to lower it; therefore you can continue to fly and land the airplane (at least in daylight VFR conditions) -- but with solenoid valves, you may be limited to the fuel in the collector tank or, with a better design, you'll be on "both" willy-nilly or feeding from one wing without the ability to switch (leading to trim problems as fuel is depleted).
Bottom line, a fuel valve that works any time you can keep the engine run is better than one that quits if the master switch gets flipped.
You would use a solenoid or other electrically operated valve, like a torque motor operated one, only if you must use it remotely and it isn't practical to operate it mechanically.
In doing so you also have to provide redundancy in the form of a manual override (such as a cable you can operate that forces the valve open following an electrical failure in the valve or upstream of it, or a sticking solenoid), or dual valves in parallel (or whatever else you can design to eliminate safety-critical single-points-of-failure).
Solenoids like to stick and torque motors like to burn out, and even if the valve is spring loaded to be normally open so it is fail-passive in theory, I wouldn't trust my life to it without some form of backup.
In a light aircraft design where the components can be made to be easily in reach of the pilot, a manually operated selector valve, or manually operated shutoff valves at the wing roots is the way to go. There is simply no point in designing an electrically operated system when a simple and robust manual alternative is easily done.
My preferred design would be a left-right-both selector right at the collector tank entrance, a separate emergency shutoff valve at the firewall operated by its own remote mechanical control that sticks out and gets in the way of things when at OFF, and ideally, manually operated emergency shutoff valves at the tank outlets in each wing so that fuel is contained within the wing if it gets broken off in a crash (that may be overkill - I've never seen shutoff valves right at fuel tanks in light aircraft, but I'd do it if it was my own design).
