The Subaru 0W-20 has a way lower viscosity Index then the Mazda oil.
221 vs 169
That means the Mazda oil is lighter on start up and operating temps. When the Idemitsu oil went from GF-4 SM to the new GF-5 SN rated. It went from having a VI of 197 to 169. It also had over 600 ppm of Moly in it. It's pretty much doesn't have any Moly now. Both will serve the purpose of the 7,500 miles oil change interval. They are both priced the same. I will buy the Mazda oil for better start up wear and maybe better MPG, seeing it is lighter at normal engine temps.