Why would you disregard the novel? That provides far more in-depth coverage, and besides, this is DE Sidious, who canonically grew in power.
Marek also never came close to beating Sidious. Sources confirm he could have never beaten Palpatine, and it's evident the Emperor held back in that false fight earlier on to goad him into joining the dark side (IIRC, even Galen admits this in the game). Regardless, the fact that Galen ended up dying from Sidious's Lightning even when Sidious himself was utterly unharmed pretty clearly shows that Sidious >>>> Marek.