Obviously if I am supply blocked and in a pinch I would use the supply add-on but unless I screw up and become supply blocked I generally won't so I'll focus this question on MULE vs Scanner Sweep.
I noticed that most people tend to say it's all about the MULE, however I tend to have a lot of success just saving up the energy for scouting as I've found that intel is EVERYTHING. Another great thing about the scanner sweep is for looking before you leap when moving in an army. To me it's just been better to save the energy for the scanning unless say I get harassed and lose some workers so I throw down a MULE to compensate.
Thoughts?