You marketing channel is the way in which you create awareness about the products and services that you sell. The way in which you market to your customer is highly dependent on who your customer is. It is therefore critical to consider where your customer will be spending their time, as this would be where you might want to reach them.
Your sales channels has to do with how and where your customer will decide to complete the transaction.
Initially, when you have just started the company you will spend most of your time and resources on sales. As you grow the company and the company becomes more successful you might find that you spend more of the companies resources on marketing.