Despite being the polar opposite in a way where it's clear things behave the same on both sides, you can't really say we could treat one 'the same' as the other story-wise because they're very different results. Sin Eaters are basically mindless zombies with some having the barest inkling of what they were like in life, while voidsent actually do retain some agency and personality, even if it's in a situation where that's not doing them a whole lot of good.

I think it's stupid to ask 'will we give sin eaters the voidsent treatment', because I think we already did; the stuff we're doing isn't new, it's giving Voidsent the Sin Eater treatment, in a way that's more suited for their traits and needs. Something they really put at the forefront with Sin Eaters is 'these were people', and exploring both the loss of personhood that entails (both with Tesleen and Halric) and the exploration and mourning of who they were (in particular with the Cardinal Virtues). With the Voidsent there's not really that notion of the individual having passed so much as gone through hugely life-changing events, so we can still interact with them as present-tense individuals, but there's still at least for some of them an exploration of who they were before.

Basicaly the main difference is that we can talk to voidsent and get some perspective from that. Talking to a sin eater would not be tremendously fulfilling.