Whatever timing happens, even if its 1 year difference between releases, I think that large power difference like we saw between PS4 and Xbone will not be present.
Xbone was strategically designed to have 8GB of RAM [they belived only DDR3 could achieve that]. That forced them to "
waste" die space with very large amount of ESRAM, which enabled
smaller PS4 APU chip to house 50% more CUs. That 50% difference was noticeable in games, and Xbox One got its fair share of 720p games [CODs, Battlefields, MGSV, etc]. MS simply fucked up at planning stage.... or like in professional Starcraft, it was a build-order loss.
This time, both will again be on same production node, both will dedicate entire APU to ZEN & big GPU. Even if Xbox Infinite launches in late 2019 with Zen+Vega, and PS4 launches in late 2020 with Zen+Navi, I don't think performance difference will be as huge as in gen8.