I think Utras are always your best option until they are literally on nothing but air units. A few ultras are useful if they have any templars/archons on the ground. I listened to goswser talk about this and its essentially the same thing he said. Ultras are even useful for running around air armies and killing bases on a map like Whirlwind.
If toss gets to pure air with templar to zone you out and you can't keep their bases in check, I don't think zerg can win.