Charging a Model 3 at 240V is very efficient, and noticeably more efficient than charging at 120V. You will pay about 12% more to charge at 120V compared with charging at 240V.
I installed a nice little power monitoring device in my Circuit Breaker Box (an IoTaWatt from www.iotawatt.com) that lets me monitor the power used by each individual circuit, and decided to authoritatively answer the question of how efficient the Model 3 LR is at charging on 120V vs. 240V. I've seen all kinds of numbers thrown around, using all kinds of proxies for actual power (Miles gained, battery % gained, etc), but now that I had the ability to measure actual power I figured I could be more accurate.
See below for more data. The forum software won't let me post this all together.