The true benefit for me is access to the 4K quality. as long as you have the respective high end equipment.
...which means that getting Premium is a leap in the dark, because you don't know if its any good till you've got it.
You can regard Premium as a paywall to the top quality, though the cost is less if you are planning to spend a lot anyway.
I cracked and paid up, because I wanted to see if 4k was worth it, reckoning that my PC should be able to take advantage of it. I worked out the cost, based on my buying habits, and bought it. Now my slow internet connection is bunged up every night reloading cards.
My view is that the extra cost is worth it, if you want to run it on a PC that can do 4k vids and you want to see iS clips at their best. To my eyes, the quality difference between 3k and 4k is similar to that we saw when VirtuaGirl went 720p to 1080p. It's startlingly sharp, though, of course, you get used to it. 3k clips start to look a bit soft.