Not to be arguing with Toiville, what works for him works for him, but I have a strong preference for a variable.
My .204 Ruger and .223 both wear Bushnell Elite 6x24s. In my group most folks spot with binoculars, then go to the gun. If your gun is wearing a high-powered fixed magnification scope, it can be a real bear to find the dog you spotted with your binoculars. With my variable scopes when I switch from binocular to rifle, I start at 6 power, making it much easier to find the dog, then zoom in. My max range for the most part is what you cited, 400 yards, and a 6x24 will work fine for that.
Another advantage for the variable is that sometimes you'll be shooting out at 200-300 yards and suddenly one pops up at 50-60 yards. If you're shooting a fixed-power 36 or anything around that, you'll be cursing that fixed scope as you pray for the dog to stay up. With a variable you simply drop down to 6 power, get on the dog, and launch him.
Also, you'll almost always be shooting in bright sun, so there's really no reason to pay extra for a huge objective or 30mm tube IMHO. If you monitor some of the shooting/varmint forums you can often find a good used 6x24 for a lot less than new.
That's my 2 cents' worth. Have fun.