How to switch "best" between two voltage sources (5V & 3V3) for a single output ("MUX for VCC")?
I would go with the transistor version.  It consumes less power, generates less electrical noise, and allows a state when the sensor is completely unpowered.  However, the design needs work.

Add pull up resistors from the Q3 and Q4 transistor bases to their emitters.  This assures a fast and complete turn off.  10K should be fine.

Decrease R3 and R4 to 360-470 ohms.  For 75 mA collector current you need around 5-7 mA of base current for firm saturation.

Decrease R1 and R2 to 4.7 K or less, again for firm saturation.

Delete D1 and D2.  The transistors provide enough off isolation by themselves, and deleting the diodes increases the output voltages to within 0.1 V of the source voltages.

You might want to add some decoupling capacitance on the switched output, but that is a tradeoff between filtering noise and adding a delay to the time it takes to switch from the 5 V output to the 3.3 V output as that capacitance discharges by 1.7 V.