As a sales person and someone who also interviews prospective sales people, I have an idea of their motivation for this question. They are looking for YOUR motivation to sell their products. Do you have a passion for the product line or services? Or, are you just looking to make some money?
First, I would ask a question. What are the commission structures for both positions? This shows you know what sales is about, making money. No matter their response to your question, your answer should be about how you can help them and want to learn from them. You can also ask which product is their 'bread and butter' or 'up and comer' or which they feel is going to lead them into the next phase of company growth.
Always respond with positive comments on both products, no matter which position gives you more financial gain or you have more interest in.
If you are really green and know diddly squat about sales, the products or the industry, always (and I mean always) express interest in learning about all sides of the industry, the company and what they are selling and how you can help them.
If I were interviewing I would also ask if moving between the departments at some point would be an option. I'm a stead fast believer that one must know everything they can about the business they are in to be able to sell it better.