User blog comment:B214/Volume 21 info & spoilers/@comment-26285797-20160715154901/@comment-24.113.252.213-20160717014854

The dynamic between Issei and Vali is pretty much the same as that between Naruto and Sasuke.

Issei and Naruto have no talent, but have powerful creatures in them (Ddraig and the Nine-Tailed Fox).

Vali and Sasuke have a lot of talent, and have special lineages (Decendant of Original Lucifer and Member of the Uchiha Clan).

Issei and Naruto keep training and training to reach a new level. But Vali and Sasuke, due to their special lineage and talents, reach that level with half (if not less) the effort, and somehow are a little stronger than Issei and Naruto.

But in the end they will likely end up on the same level.