Both locations are good for a solo trip. If you are looking for experiencing snow-capped Himalayas then you should go to Sikkim. If you are looking for misty mountains then you should visit Meghalaya
Meghalaya is full of nature especially now for after rains have left the place more lush than needed. Transportation is quite easy in Meghalaya and stay is available abundant. Sikkim on the other hand is more of mountains, hill slopes and landscapes. Covered by snow peaks all through winters, transpotation is easy to reach too. As far as stay is concerned there are places to stay too., My choice would be Meghalaya.