My instinct would be your option (A) because, from what you have said, Products are Services are different entities. What other attributes beside Description does a Service have and what do you want to slice your RetailSales Measure by? If you want to slice by Service distinctly from slicing by Products then that would also point to having separate dimensions.

Also, if you creating Hierarchies (and you should to improve performance and to increase the usefulness to business users), it will be easier to create meaninigful hierarchies with separate Product and Service dimensions.

Regards

Lempster