What should I take account for when evolving?

So, I've got this ghastly with the IVs 15/14/4 and I'm wondering if it's worth evolving? As to my knowledge the stamina is only adding a few more HP to the Pokémon. This is just the thing which makes me wonder if the gengar would be any more fragile because of a lower stamina? Cause isn't it a matter of each point in stamina increasing HP by 1 point?