There is exactly a thread on this not too long ago if you search for it. The OP came in with the KS7 in mind but the Costco HIS recommended the Phonak Brio 2 to the OP. I think the OP obliged with the HIS recommendation and trialed the Brio 2 first. But she had intention to also trial the KS7 shortly afterward as well and may go with the KS7 if she doesn’t find the Brio 2 significantly better.
I guess everybody’s hearing loss is different and the KS7 may work better for someone and the Brio 2 may work better for others. Your dad can follow the Costco HIS’s recommendation and trial the recommended model first, but if that’s the more expensive Brio 2, I don’t see why your dad can’t also trial the KS7 as well to decide for himself.
That’s the great thing about Costco. Their trial policy is so generous that there’s nothing to lose but time and maybe tying up some $ momentarily so you can try out both.
You can probably PM the OP of that other thread to get her own opinion on it. And find out if your dad’s audiogram is similar to her or not.