The two apps look very similar. The main difference is that Candy Rack is a pop-up that is triggered by clicking the Add to Cart button, while Candy Cart pops up when customers hit the Checkout button (right before the checkout page).
There is also a technical difference - Candy Rack uses the standard Shopify checkout and applies standard discounts (if any). Just be aware that Shopify allows only one discount per order.
On the other hand, Candy Cart uses draft orders to add upsells/cross-sells/gifts to orders, which allows us to add multiple discounted upsell products to the orders.
For full details: Candy Rack vs. Candy Cart – What Are the Differences?