I have the slip clutch on my mower, dad has a shear pin on his mower.
When we first started cleaning the farm, he would break a pin about every month (three in one day). We were mowing in areas that hadn't been mowed in 10 years. Now that we have the majority of our mowing areas well defined, he may break one during the season.
The slip clutch I adjust on the first mow of the season, add 15-20 minutes of the start time. The rest of the year no adjustment.
So, if you are mowing in an area that is well defined and no hidden obstacles, then pin is fine. If you are in the process of clearing an area or mow in areas that you are not familiar with, the slip clutch saves time.
Just my two cents based on our experience.