While this more immediately harms the economy, after several minutes, it would usually have been better to kill SCVs
Just testing in game,
135 minerals take about 3 minute, 270 takes 6 minutes give or take a few seconds, so 45min/minute.
The Mule of course does do 270 in 90 seconds, so 180/minute.
I've often heard people say 'A mule is like 6 SCVs!' because it carries 30 minerals vs 5. but it's interesting to see that this is not true. 1 Mule = 4 SCVs
So, back to my original point.
Say you Swoop in with 4 Phoenix, You stumble upon 4 mules and 4 SCVs at a new expo.
Assume the Mules are on average at 50%
If you did nothing, in 90 seconds, they'd have: 810 minerals
In 3 minutes, they'd have: 1080
in 6 minutes: 1620
in 10 minutes: 2340
in 15 minutes: 3240
So that's if you just flew by
Now, If you killed the 4 Mules:
In 90 seconds: 270
3 minutes: 540
6 minutes: 1080
10 minutes: 1800
15 minutes: 2700
If you killed the 4 SCVs
in 90 seconds: 540
in 3 minutes: 540
6 minutes: 540
10 minutes: 540
15 minutes: 540
Now count in the fact that the SCVs cost you 50 minerals each...
Basically my conclusion here is unless you imminently need their econ hurt immediately, you are better off choosing the SCVs first. If the game goes one 10 minutes past your harrassment, one SCV is worth 500 minerals more than the single MULE