PayPal Express Checkout is the normal PayPal flow, which is available for our older and new integrations (Drop-in & Components).
PayPal Express Checkout Shortcut (PayPal ECS) is essentially another PayPal button that shows outside the normal payment flow, i.e before you go to cart/shipping details etc. This is currently only available for our older integrations, with the redirect flow.
In terms of functionality, PayPal_ECS offers the capability of having PayPal outside of the main flow as it is presented earlier in the shopper journey to collect the details and get redirected.