Night Shade doesn't hit normal types, Seismic Toss does & vice-versa. Now take in consideration that mew doesn't want to stay in on Ghost types to begin with, whereas it might actually want to do just that against several normal types.Actually a pretty obscure reason. There is no Pokemon that can learn Imprison and Softboiled, but there are Pokemon that learn Roost and Imprison. 99.99% of battles they will be the same thing.
Also what is the reasoning behind Seismic Toss over Night Shade?
Anyways, three years later and I finally got to SR'ing Xerneas. Right now I have a Timid one with the following Ivs: 29/xx/31/31/29/31. I do not expect it to have any trade value, I just want it to be competitively viable but I have no idea if it'll miss those 2 points in HP & SpD. Sooo, toss or nah?