iMessage is more popular. You want people to use Signal, but if they don't want or cannot use that, iMessage is his preferred alternative. For a similar reason, I use Signal and WhatsApp. (All 3 are E2EE.)

Nicholas Weaver covered iMessage's strengths and weaknesses before:

(20150804) iPhones, the FBI, and Going Dark [1]

(20150806) Nicholas Weaver on iPhone Security [2]

(20150908) Apple's iMessage defense against spying has one flaw [3]

(20171211) Is Apple Fully Complying With iMessage and FaceTime-Related Court Orders? [4]

I also remember a technical analysis on iMessage by either Matthew Green or Nicholas Weaver.

[1] https://www.lawfareblog.com/iphones-fbi-and-going-dark

[2] https://www.schneier.com/blog/archives/2015/08/nicholas_weav...

[3] https://www.wired.com/2015/09/apple-fighting-privacy-imessag...

[4] https://www.lawfareblog.com/apple-fully-complying-imessage-a...

iMessage is end-to-end encrypted, and a good bet that the app publisher is safe. Apple has a strong record of strict security and anti-government-snooping policy.

iMessage is end-to-end encrypted as well.