Both are about a past love in a different life - both about finding the "reincarnated" one in the present - both have similar things - like a tomb being found decades later... clothing, hair, letters ectra... I personally enjoyed 'Dream of 400 years' more - as there was much more to the story and the development and end were much better.
Again, similar theme. Love from the past reincarnated in the future, both dream same dreams and are searching for clues to who that person in the dream is. and again, I thought 'Snow Lotus' was the better of the two. But if you enjoyed one - you might enjoy the other. Snow lotus has a better ending.