Hi,
Both states offer utmost natural beauty as well as refreshing and magnificent experiences to travellers. It is difficult to deem any one as better. I recommend going through the experiences of other travellers in these states and deciding which destination suits you more. Attaching a few helpful articles below.
You could also try visiting both the states if you have the time!
You must travel to Meghalaya in June. Shillong, Cherrapunji, and Mawsynram are some of the must-see places to visit in Meghalaya, at this time of year. June in Meghalaya is a lush green landscape, with amazing waterfalls. You can explore locations like Dawki, and Mawlynnong, in addition to Shillong, Cherrapunji, and Mawsynram. These Meghalayan locations offer a variety of moments, from exploring amazing landscapes to becoming part of the rich local culture (Read more at https://wanderon.in/blogs/places-to-visit-in-meghalaya-in-june) .