So lets group their special things.
First of all, they're both immune to ground. Win-win.
Like said in the comments, Rotom Heat has many resistances- Fire, grass, flying, ice, bug, steel, and fairy- but with two weaknsesses, water and rock. Rotom Water/Wash, however has only one weakness- grass, but has many less resistances- steel, fire, water, ice, flying, and ground.
They both have the same stats, and neither get any moves to cover their resistances- Unless you count Sunny Day and Will-o-wisp. But all Rotom can learn that. Overheat has 130 power and 90 accuracy, but with the special attack drop, it's kind of impossible to use it 2+ times without getting stat boosts. Hydro Pump's power is 110 and 80 accuracy, so it's a bit of a loss.
So overall, it pretty much depends if you're looking for someone to resist attacks, use overheat, and switch (rotom heat) and someone to just to Hydro Pump and troll people who don't have any grass moves on their Pokemon, go for Rotom Water.