Neither of those examples are really cross compatible. The first one will show a high dpi image on IE, the second will not show an image at all on a non srcset device.
https://caniuse.com/#search=picture
https://caniuse.com/#search=srcset
The reason the srcset is in there is specifically for seo. The time of loading is negligible, but using it helps with seo in the sense of the internal tests that Google uses to determine site speed as a ranking factor.