Udinra All Image Sitemap plugin was received well by WordPress community. With time it became highest rated , most downloaded and installed Image Sitemap plugin which is constantly updated.
Pro plugin was created with more features but keeping user experience in mind. The code base of Pro plugin and Free plugin are different.But the aim was same to create Image Sitemap from your images.
The free plugin receives code optimization and other benefits from Pro plugin as well.So it is two way communication between Pro plugin and Free plugin.
Both are evolving with time and Hope you get benefited using them as well.