Sikkim is very pleasant in august. Rains, downpours are seen full of august. Some best views of valleys, lush greenery is visible all through august. Meghalaya is more of rains in august. Cool waters, cool air, fog and lovely rain drops are seen all over Meghalaya. My suggestion would be visiting both spots, provided you are an avid monsoon traveller. Travel to both spots in august has lots of pros and cons to follow. Kindly do a weather check before travelling.