Quantity Breaks applies discounts when minimum quantities of specified products are added to the cart. For example, X quantity of Y product in the cart will discount each quantity by Z. You can allow customers to mix and match products where the quantities are combined to trigger a quantity-based discount. Quantity Breaks will show a grid on the product page of selected products that illustrates what quantities result in which discounts.
Product Bundles applies discounts when all specified products in a bundle are added to the cart. For example, if product X and product Y are in the cart, discount Z is applied to each of them. Quantity is only a factor, each product in the bundle set in the cart is discounted. If one product of the bundle has more quantity in the cart then the other bundle products, that extra quantity won't be discounted (that extra quantity isn't part of a completed set). The app will output a bundle widget on all product pages of the products that are selected to be in the bundle, and the customer can add the entire bundle to the cart through any product page's bundle widget.
It's important to note that with Product Bundles, the same products or collections can't be bundled with each other, whereas with Quantity Breaks you can add multiple instances of the same product to the cart to trigger a quantity-based discount.