based on feats, solar ftw.
not only did he create and destroy numerous universes, just to see how certain events would play out - but he also destroyed the whole of valiant AND image in one fell swoop [a multiversal feat fo sho].... and he accomplished that feat at half power to boot.
whereas extant created a single universe which merely threatened the rest of DC [a multiverse.] though i will say that if extant would've possessed a complete worlogog, instead of the incomplete 'gog he had during the JSA issues, i doubt solar's attacks would have much of [if any] affect on him..
but as it stands... yeah, solar wins.