It will be hard to compare an external, running at (now fairly low res) full HD - to the much-sharper internal retina screen.
Your display has a max resolution of 1920 x 1080, and your MBPro supports an external display, running at 5k resolution (5120 x 2880)
Just to show the numbers:
Your HD screen displays the image on 2 million dots.
A 4k screen, at the same physical size, displays on 9.4 million dots.
Your built-in screen is actually slightly higher density than a 5k screen.
5K screen shows the same image (same dimensions on the screen) using more than 14.7 million dots!
Actually, your MBPro will support 2 5k external displays, or up to 4 4k displays - so it would support the use of a much sharper screen than what you have now.
Some users report that the curved screens (such as yours) are great for gaming, but not so good for text. I don't know if that's true or not, but the HD resolution on a relatively large screen will be more blurry than a 4k (let alone a 5k) screen.