From my point of view I would say Mahabaleshwar as Lonavala is very hyped and that is why it is crowded and expensive. Mahabaleshwar is beautiful along with Panchgani and pocket friendly .
Mahabaleshwar is place for getaways, you can spend your weekend there normally as Normal hill station . If you want to enjoy holi with everyone you should go Lonavala
Hey!!
both places will be similar for you since there is no Holi celebration as such but some events manager can organize Holi event , you can take part in them, for sightseeing purpose both places are good to visit but best time to visit them is rainy season
All the best for your journey!!
If you are looking to experience Holi in the best possible way, skip both above destinations & go for Barsana or Vrandavan. This will provide you lifetime opportunity for the Holi