I really love my Stout Conicals but I think butterfly are overkill. The Ball valve is much better on the racking as you can better control the flow. Additionally they are very easy to clean so the CIP of a butterfly is not worth the cost. I have replaced the butterflys with ball valves and use the butterfly on the outlets of my MLT, HLT and BK. They work better with pumps.
Bigger is better. You can half fill a conical and still be ok. Better too big than two little. You may need the head spave