If price alone is what matters, go w/ the real thing (mined).
However, with mined diamonds, price is directly proportional to quality. Lab created diamonds are generally close to the more flawless mined varieties (colorless mined). So if it's about having the nicest stone, the lab can do as well as the mined.
Women like to show it off to their friends, who certainly won't know its synthetic (its diamond, after all, just made under controlled conditions). They'll see a perfect stone, cause it sure won't be stamped "lab made." Yeah a trained jeweler might be able to tell, but who cares? Jewelry is worn to be noticed, not scrutinized, after all, right?
Which matters more? I'd personally be willing to buy a lab diamond as a gift, but I'm not to the point where an engagement ring would work that way. My mind might change later though.
EDIT: I thought moissanite was the lab created diamond, but the OP is asking moissanite vs lab diamond. I have no idea what moissanite is, but the lab diamond is really affordable and supposedly of excellent quality.