In my personal experience, I find that if I want to improve my mechanics, I run gunboats.
When I run the shotty, I find that I have a habit of 'wasting' my rockets (going for stupid shots that I shouldn't be going for in actual games), but when I'm running gunboats I tend to think more carefully about ammo management. This is probably not be the same for everyone, but it happens to me a lot.
Also, I rarely run shotty in actual 6s scrims/pugs/matches, so there's that too.