As far as I'm aware, the Karnali has a rounder hull and softer edges than the burn. And the ammo is more of a planing hull with harder edges (like your playboat). I'm pretty sure the burn is a little in between the ammo and the karnali.
I think it really depends what kind of rivers you are paddling, and your paddling style. For me, I have an ammo because I mostly paddle my plaboat. It's easier for me to jump into something with rails and a planing hull and paddle than to get a full displacement hull. Also the ammo has plenty enough volume for the rivers I paddle.
Just something else to think about. I don't know what sort of rivers you plan to paddle, but if you don't really NEED a full on creeker like the karnarli or burn, the ammo might be an option. If your paddling tight/technical stuff, something shorter that is easier to turn will make your life much easier.
Your best bet would be to demo the boats first if possible. If not, just buy one at a discount, if you don't like it, sell for profit or what you paid haha