I have found that the experience is totally random. And as such, I always spend the extra time to physically check out and get a hard copy where possible. Last week I got a paper receipt, and there was an email in my Inbox before I even got outside to catch an Uber. And also last week, at a second location, I had an email *BEFORE* I checked out.
Other times, assuming the promise of an email, I never get the email, and have had to wait days before it's available on the website.
So, now, I always get a hard copy. If the email comes before I do my expense report, then great. I don't have to take a picture or scan.
But I always have that for backup.
If Marriott wants to champion saving paper, then they need to really do it.