Both are possible. It depends on your needs and how many employees your client has. If your client is relatively small (maybe up to 50) people, it can make a lot of sense for you as the designer to buy it and offer it as a service to your client. You could then use the number of templates that come with your plan for your different clients as long as the number of employees using it still fit within your plan limit. If you have a larger client (with more than 200 or so employees), then you would use most of your plan limit on one client, so it may be best for them to purchase their own account and then give you access.