I have both of them and I reviewed both.
The vibrations are similar and of course the large is stronger. It just depends on what you want to use it for as far as which one is better. I personally like both. But if I were to use it for clitoral stimulation, I'd prefer the medium just because it's smaller. I bought the large for body massages. That is why I don't have the Hitachi or other wands, I don't want to hold something big.