Never played a Cast.Lilly or Mengascini so I can't offer much by way of experienced advice but my limited knowledge leans me towards the faster hand made reed instruments. I love my Cast.Tommy 3VC for its right hand speed and 3rd bass reed but it lacks the full bass chords of my Dino Baffetti.
All in All it's nice to have more than one coz they all have their own different characters and it's nice to play horses for courses. I particularly like to use the Hohner Polkerwork for song accompaniment , probably because the wet tuning hides the multitude of sins in my voice!!!
Having read what I have written I realise it is no help whatsoever so I would go for the Cast. Lilly if only for the resale value.
Spot