I find a good mix of simple and complex words is best. I don't like to use all short, simple words for fear of sounding slow or unintelligent. Then again, too many complex words can come off sounding pompous or like you're trying too hard. Therefore, an even mix sounds intelligent but accessible. And with an even mix, even if someone doesn't understand one of the words you use, if it's the only complex word in the sentence, they can probably figure out what you're saying from the other words, saving you the time of having to explain yourself. So overall, don't avoid using big words just to stay simple, you will sound much more intelligent and professional, even when not specifically trying to.