Actually neither of them should be your determining factor. You should sit facing where you believe where someone may be coming from to greet you. For example, if there is only one hallway flowing into the reception area, you should position yourself so that you can see someone coming down the hall before they actually approach you.
The next concern, aside from where to sit, is then to determine which piece of furniture you should sit on, a sofa or a chair? Although the sofa is more appealing, the chair is a more practical choice.
When sitting on a sofa you may tend to relax quicker than if you were sitting in a chair. And if the sofa is particularly comfortable and sits low, you might find it difficult to get up, making it awkward to gather your belongings while simultaneously trying to greet your client with confidence.
Remember, the first view others will have of you most likely will be where you are seated. For this reason, your sitting posture has to be as effective as your standing posture. Good posture translates into a strong presence.
To help you sit with good posture begin by sitting on a chair with your back about 3 inches from the back. Now take a deep breath and let your air out. Then, without thrusting your chest forward, lift your body by pushing your waist forward as you did in the standing exercise. This will give your posture energy and help you project an air of confidence.
Practice this until you feel natural and look natural. If you appear to be stiff, then you need to work on it until you find a posture that is comfortable and feels natural to you.