The verdict
If you run a B2B company with a sales team, a CRM that Marketo connects to natively and someone who will own the platform, pick Marketo Engage. If you run a Shopify store and want text messages to bring in repeat revenue, pick Postscript.
I would not compare them any further than that. A Shopify brand has no use for lead scoring and sales alerts, and a B2B team has no use for abandoned cart flows. If you are a small B2B team without a marketing operations owner, I would skip both and look at HubSpot or ActiveCampaign, which are lighter to start with. If you are a store that does not use Shopify, check the fit of Postscript before you go any further.