I have both of them and I reviewed both.
Medium:
link
Large:
link.
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.