i'm in your conscience
Just evolve it. You want the better stats I think, and I don't think Eviolite Haunter is a thing...or a good thing. Cause being bulky isn't what Haunt or Gengar is about. It's about fast hard hitting special attack, and you're gonna get that more out of a Gengar than you would a Haunter. But if this is just for in-game, it doesn't really matter.
I don't recommend Haunter for gen V competitive though, but that's a topic for another section. Keep in mind, if you want help with competitive stuff, you're gonna wanna try asking in Battle Center. You will get better results asking there than here, plus if you're sending it to gen V, this is the gen III section lol. Though I'd recommend getting a completely new Haunter to use competitively rather than one from an in-game playthrough, but maybe that's just me.