No worries, you don't have to justify it. Just buy the 3 if it is better fit for you; it will be for most people. But here are things some prefer about the S:
Considerably quicker in any trim; and P100D available (upcoming 3 P model may match it, but will change the cost comparison)
AWD (also coming for the 3, although it will narrow the price difference)
Better warranty
Twice the cargo space
A large hatch to access that space, and it's powered
More screen space, and one of them in front of the driver
Adjustable air suspension
Better ride
Better looks
More range in any trim
Includes a key fob
Center console even with the base model
Dynamic turning lights
Free supercharging generally available
CHAdeMO adapter available
Bioweapon defense mode available
Pano roof available
Rear-facing seats available
And, I suppose, quicker availability.