Depending on what kind of product you are selling one of the two options might be more appropriate. As always testing the hypothesis must be done to make sure we made the right decision. The difference:
– Add to cart: I press the button and it adds the product to cart and sometimes it redirects me there. On the cart page I might get another offer or an upgrade of some sort.
– Buy Now: I press the button and it takes me to the checkout page so I can buy this as fast as possible without any other up-sales or addons.
I would use the “Add to cart” option on a regular product, on a shop where I sell a lot of other products and I will give the customer options to upgrade his purchase, upsells and addons. This would be a longer funnel sale.
For me if I sell a product at a promotion and I want to sell a lot of it I will use the Buy Now option. I want the user to buy the product quick and I will add a sense of urgency to the sale, create a bit of scarcity. For example if I have a promotion on Black Friday at a great product, Buy Now would be my option.