I think the power difference between them will be so negligible, that it may not make a notable difference this time.
The reason the PS4's power advantage over XBO became such a big deal wasn't just because it was more powerful, but because XBO games commonly didn't reach industry standards of 1080p output, etc. If they did that, it wouldn't have been such a big deal.
And whatever difference there may be between PS5 and XB2, it's probably not going to be something like that this time around.
As for which will be more powerful, that's hard to say. If the current rumors are true and PS5 is more powerful, Microsoft may respond by increasing the specs further.
It may come down to which one launches last.