That Mega Blastoise decal is awesome. @_@
As for the Mega Mewtwo placement, I'd say go for the top design, and maybe include his lower torso on the back of the 3DS like how they designed this Charizard 3DS to incorporate the entire picture if you completely open the 3DS:
And I'd also add that an outline stylized design would really be the most eye popping. Like you see with that Blastoise 3DS, as well as Ktay's designs and this Charizard design.








