because misses don't count. So, if you are not good with an AR-15, use what works best for you, but if you want to train with or use the best tool for the job, train with an AR-15.
If you are familiar with the AR15, or you have just as much experience with an AR-15 as a shotgun, I would opt for the AR-15 for sure.
Here's the same article on a different link.
https://www.ar15.com/archive/topic.html?b=1&f=5&t=1537272