I would have a major problem if ANYONE I was involved with didn't show their emotions, especially in this world. I mean, are we not a sect devoted nearly entirely to trust? And if one cannot be trusted with anothers emotions, how can one be trusted with anything at all. Emotion is a basic, primal thing, and neccessary. I would not want someone who was a blank cold slate, nor would I care to be one. A Master/Mistress must be caring and compassionate, as well as dominate and agressive. I mean, if all you are is horny, that's really not a relationship, as far as I'm concerned.
On the matter of a low after a session, I would have to say that, while it occurs very rarely, and usually only because of exhaustion, I likely suffer far more of that than Master does.
I don't think it is easier in either position to show emotion, though it is more expected of a sub.

In other words. Trust eachother with everything you have, because if you don't, you'll have nothing.