The base attack advantage of Apocalypse sword over most dark SRs with Assault (M) ought to normally surpass the 3% assault difference. Dark's a bit hindered in base attack relative to other elements because of eidolons, so you need to make up for it via weapons.
Besides, over the long term, even when a SR would ever so slightly edge out in per hit damage, Rush (M) should come ahead in total through the extra hits.
It also gives you an option for sword using souls if you ever want the burst effect (dispel one buff I think).