Meghalaya, the “abode of clouds” is a paradise for nature lovers, home to a beautiful, mesmerizing sky, filled with huge hills, valleys, numerous lakes, captivating waterfalls, pitch dark caves, and sacred forests with panoramic views, foggy hills, terraced slopes, cascading waterfalls, and serpentine rivers mesmerizing your soul. You can visit Shillong, Cherrapunjee, Tura, Jowai, Nongpoh, Baghmara .
Whereas Sikkim is known for its beautiful scenery, green meadow, crystalline lakes and towering mountain ranges with the coexistence of Hinduism and Buddhism with monasteries, rhododendron groves and thick woodlands, orange and apple orchards, sanctuary of wildlife is your go-to for trekking into the Himalayas and river rafting on the Teesta and Rangit.
Both are beautiful places in it is own unique sense. But for me Sikkim is more preferable than Meghalaya. You can have your own take basis which place seems more panoramic to you.
I would prefer Sikkim, If you are a travel freak, Sikkim is the best place for you travel long to visit tourist spots, The travel would be so breathtaking and fascinating, It has the worlds highest lake (Gurudongmar Lake), Nathula Pass (Indo-China border), Tsongmo lake (try the rope car between the ice capped mountains) and the Yumthang valley. All the tourist destination are about 100 kms from Gangtok-Sikkim, the way filled with sky heighed mountains and deep valleys would be really a mind blowing experience. .