When should I evolve my Haunter?

I'm on vacation at the moment, and I won't be able to evolve it until I get back. I realize that Gengar and Haunter learn all the same moves at the same levels, however I'm worried that if I evolve the Haunter at a higher level it will be weaker than if I evolved it at level 25. Can I go ahead and train it? Or should I wait until I am able to evolve it?