The thing is, while Rotom-H has very useful niche, it gets walled hard by TTar, Garchomp, the new addition Zygarde could jump in and just Coil all over it's face, and a few other things check it really well, especially after HP nerf. But weather nerfs and Paralysis immunity are great buffs, and it has amazing dual stabs with the capability to wreck lots of mons. It's one of the only Rotom forms (cough looking at you Rotom-Fan coughs) that really benefits from it's ability in Levitate, giving it yet MORE opportunities to switch in and wreck some shit. It has lots of utility and uses, and I predict both Rotom-W and Rotom-H will stay in and around the same tier, namely OU or UU. Both of the forms have very valuable niches, it just depends on which one will be most commonly useful. H seems like a great addition to hyper offensive teams, or just offensively oriented teams in general, whereas W loves being bulky and annoying.
On a sidenote, Rotom is so adorable in X and Y xD He's so small!