It's difficult to dictate this battle by powerscaling if you accept that Valkorion isn't a sith. I suppose you could argue that Sidious's "the greatest master of evil ever to use sith power" accolade also covers Valkorion because he is a master of evil who has used sith power in his past, and the semantics of the statement technically applies to his having used sith power before, but whatever. We know from accolades that Sidious > Vitiate, and if you take the publisher's blurb seriously, that Plagueis > Vitiate, which would create some buffer for Sidious being above him by a fair margin (he grows more powerful upon Plagueis's death, and then presumably grows stronger for a decade), but then this would all depend on the extent to which Valkorion is above Vitiate. I would say not by too much. Most of Ziost likely went into just healing Vitiate. He probably runs into diminishing returns from Nathema-type events; otherwise, why didn't he just continuously replicate it and make himself god-like?
From feats, it's tougher to definitively put RotS Sidious above Valkorion, as I think you can with, say, RotJ or DE Sidious, because given his situation he can't really show off his Force abilities in a grandiose manner that frequently. I maintain that his and Plagueis's unbalancing the Force is significantly more impressive than anything Valkorion has accomplished on his own power. What Sidious does have the advantage in is combat feats - his blitzing the B-team, matching Yoda and tooling Maul and Oppress impresses me more than, what, one-shotting Darth Marr and knocking out Arcann? But it's tough to get an upper limit on Valk's abilities.
I still give this to Palpatine because of his abilities in close quarters, which mean that he likely wins the confrontation if their Force powers are relatively break even, and I see no compelling evidence to think that Valkorion's are so amazing as to win from a distance outright.
My personal head canon puts Valkorion as an equal to Plagueis in raw power, but that's just me.