I like mine long because I can wear it in many styles and it makes me feel more fem. I love my hair and I think it's my best feature.
I've had all lengths from pixi cuts to bobs to layered long. The latter works best for me.
Unless it's super short like a pixi, I feel it is more high maintenance.
I feel henna will be a waste of time and money. If you want to be consistant with keeping the grey covered I would go full color. It's best to do it now before people notice the grey. That way they'll never notice you color.