I'd rather save the option than worry about 20 days of service time and salary.
If he was close to a service time line that might be different.
Sano came into the season with 3 years, 66 days service time. By May 1st, he will be up to 100 days. He needs to reach 172 days to hit 4 "full seasons" service time.
If you were willing to keep him down from May 1st until July 21st, you could gain an extra year of control (through 2022). If we think he still has work to do, to get ready for 2019 and build upon his 2018 reset, maybe that would be more valuable than keeping an option on him for 2020 but only controlling him through 2021?