With the picture of the sky it's pretty hard to judge since they look like it's a different time of the day in each picture. But I do believe the modified one is barely, diminutively smoother looking. It is, but not enough to brag about. Assuming that they were different times of the day, I was looking at the clouds to compare the two. And as for the picture of the ground, it's noticable. I can see it, nothing major but still an improvement.
They were at practically the same time, it was at sunset, so the sky colour was changing, and I dunno about you, but I can CLEARLY see the difference between the two resolutions, in my example and Darknut's, size does make a difference, but 1024 usually works fine, so it should be a default.
Here's my set of personal rules:
512 - Minimum size
1024 - Default size
2048 - use only on really big objects
4096 - hardly ever use, only on objects that use up a huge about of screen space, like larger than the screen.