Postmark has way better visibility into what is being sent. You get per-email history/status, so you can easily tell if something is queued or bouncing. Trying to debug errors on Sendgrid is less fun.

On the other hand, postmark is a bit more flaky - I track delivery RTT and had had a few cases in my first year with them where Postmark had > 30m delays (and no outage reported). Sendgrid has always been bombproof on delivery times over many years of usage.

I still prefer Postmark on balance.