Electrical – SPDT using transistors to switch between two seperate voltage sources

You get the local ground above the universal ground. Which generally speaking is bad.

The main reason is it can cause signals generated by the circuit to interact unpredictably with circuits connected to the "true" ground. You also need to remember that ground is not universal when expanding the circuit. Generally it's commonly accepted to disconnect the positive (and/or negative) rail and not ground.

The simplest circuit would be

^{simulate this circuit – Schematic created using CircuitLab}

where the LED is the input to your optocoupler.

Note that I've reduced your limiting resistor, thereby increasing your LED current. I don't see any reason to run your optocoupler at such low current levels.

Because of the optical isolation between input and output of your optocoupler, there is no need to keep the LED at a fixed voltage as you did on your original. It can float perfectly well.

Also, your use of a p-type MOSFET for driving your load is only necessary if you need to keep the load grounded. If the load can float, you'll get somewhat better price/performance by switching to an n-type with the obvious circuit changes to accomodate it.

EDIT - More importantly, your simulation trace fooled me into not looking closely at the output section. Your simulation is clearly not accurate. With the values shown you will only get about 12 - 14 volts across your load. If you wish to keep your present output configuration, with a grounded load, and you want something like full voltage across the load, you MUST replace your MOSFET with a p-type. Since you've used an IRF510 in your simulation, you can use its "twin" the IRF9510, although there are plenty of cheaper, newer part that will do the job. Then your output will look like

^{simulate this circuit}

and this will give you full voltage across your load.

Note that I've left out your gate capacitor, since I suspect you don't need it given the intrinsic gate capacitance of the MOSFET and the large gate resistances you've specified. If for some reason you need a very slow turn-on of your load, by all means put it back in.