In terms of culture, Twilio would be a better fit, given their focus on developer evangelism in user acquisition which is similar to Segment's strategy. In fact, if you look at Sendgrid, which Twilio has acquired 2 years ago, it also adopts a similar strategy.
Anecdata, but my experience with Sendgrid's support during a production outage was that to get a response from a human being we ultimately had to reach out to somebody on LinkedIn. Everything else via official channels just received automated replies promising responses in x business days. Also FWIW while our production e-mails weren't being sent, the Sendgrid API was happily returning HTTP 200 success responses.
I agree that it definitely was a breeze to integrate with though.
We had one emergency where someone figured out how to mass mail through our app (flawed "invite via email" feature), we were able to immediately reach support through the live chat feature and stop some 200k spam emails from being processed (they were in their queue by the time we noticed). Saved our butts.
Unless you try to send multiple requests to it's API at the same time. We've had to build in retry with random backoff into our mail sending helper because SendGrid's API doesn't like multiple requests at once.
Acquiring companies that are adjacent to your core business makes total sense in adding value to your platform, and not acquiring core functionalities allows you to claim that competition is getting stronger.