Signal is all about privacy. It trumps everything else. There are no "mainstream" payment options that align with their privacy requirements.
It's not a 1:1 comparison. They both want to achieve the same goal(payment integration) buy have vastly different requirements.
I'm in the cryptocurrency space though I don't know too much about privacy coins. However, I've seen some abstracts of studies auditing the true privacy of Monero. Specifically, it can de-anonymize users who make multiple transactions to the same recipient due to its design.
Now, I haven't personally looked into Mobilecoin deeply, and am highly skeptical at the moment. But I do think there's a valid reason to shy away from Monero based on the above, or due to it still being PoW (which is environmentally unsustainable). Signal does seem to care about privacy, and perhaps ethics to some degree. Whether or not it's "proven" (in the sense of having a large number of people using it) is quite meaningless. I really like that the CEO of Mobilecoin, who has been active in engaging with various communities, has been getting a ton of very rigorous questions here in HN, and I don't think his responses have been illuminating enough to instil trust. But I don't think it's fair to write it off as a good choice either. Presumably, Signal has access to information about Mobilecoin that we don't (which is a bad smell for a cryptocurrency, but I do hope the information about who owns how much Mobilecoin becomes public soon enough).
I'm also very interested in what the implications of Signal being a non-profit are here. Are they required to publish information about their dealings?
As to Moxie's involvement in MobileCoin it does make sense that he would push for something that has a fast transaction time. I wish they were more open about what they were doing (AMA got canceled) but I understand they are trying to avoid legal pitfalls (like Telegram did originally). But it is still early so time will tell.
Strong disagree with this claim. Signal exposes your phone number to others when you chat. I can add random phone numbers to my address book and find out if those numbers are tied to a Signal account and get to the profile even. Telegram hides phone numbers by default and also has settings to prevent such enumeration, wherein you cannot know that I’m on Telegram by adding my number to your address book unless I also have your number in my address book.