What is better for my Macbook's Pro 13' 2015 battery life: 1) Loading it to 100% and then using it to ~20%, loading it again OR 2) Using it plugged in to the power adapter all the time?
I would recommend using your battery at least a few times a week, and recharging it afterward, even if it is only for a short time.
Apple's recommendation would be to charge and discharge your battery at least one time per month. I wouldn't go below 20% discharge for the best battery life. Apple also recommends that if you're storing your Macbook for an extended period of time, to charge it to only 50%, and powering it off afterward, for the best battery life.