To add my two cents, I love the idea of the free software subscription, and I much prefer it over “fund your app”.
I think it fits better for a few reasons. Software development, especially high-quality development, is often (1) expensive and (2) boring.
Say, 1,000 of us subscribed to PureOS, at an average of $10/month. That would be a great success! And I suspect higher numbers than Purism actually has. Still, that amounts to $120,000/year. In the United States, especially after costs beyond salary are considered (payroll taxes, health insurance, etc), that might not be enough to hire even one excellent and experienced software developer to work on an operating system. Ideally, to push freedom-respecting mobile software forward, teams of top developers should be working on it. We need a lot more subscribers before we get too hasty in complaining about a lack of results.
Regarding the “boring” factor, although it would be nice for a person to be able to put $10/month in the jar toward their favorite app working on mobile and have it be meaningful, this is likely unrealistic. Purism does not really have expertise in XYZ app and its corresponding codebase, development team, etc, and as mentioned above regarding cost, it takes a lot of $10/month before it induces much high-quality software development. Like, if subscriptions are split amongst 20 different apps, and then Purism doesn’t have enough earmarked for any individual app to hire even one person specializing in the development of that technology, then that will be an ineffective system.
While it isn’t as flashy as “look at all these cool apps we are working on!”, focusing on more foundational aspects of the operating system, as Purism already has done, will lead to steady, boring progress, that will ultimately go much further and accomplish much more.
Also, Purism doesn’t have control over other projects, so there’s no guarantee they’d be able to contribute for the benefit of PureOS users anyway. Signal messenger is a perfect example. Many of the frustrations for Purism people related to Signal actually are not caused by a lack in Signal development resources (there is no such lack), but actually by anti-features intentionally created by Signal. For example, Signal tries to make either iOS or Android necessary for registering, rather than allowing a standalone GNU/Linux client. Purism could try to contribute to a standalone GNU/Linux client, but if the Signal project is not cooperating, or is even working against that effort, it could be a wasteful pursuit. So, what did this mean for “fund your app”? Signal was one of the top funded apps in Purism’s previous campaign, and what did Purism do to benefit any kind of native Signal running on PureOS? Basically nothing, and that is probably for the best, because Purism doesn’t really have the control they would need to make Signal work for lovers of freedom and free software.
No, the subscription to PureOS is better, and as long as Purism starts to produce results again (hopefully coming soon!), more and more people should subscribe to make mobile GNU/Linux super awesome 
P.S., one suggestion I have to improve the PureOS subscription model: Purism could make a commitment that all (or almost all? 90%?) of the money from subscriptions goes to PureOS development. I think that kind of financial transparency could be very motivating to people. Like, I would know for a fact that if I subscribe, then my money is going to boost Free Software in some way. I think people won’t be as likely to subscribe if they think Purism might just pocket the money or spend it to buy more hardware inventory or something.
If we want Free Software to progress, we all need to pitch in and help 
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) I quoted the specific reasons in my reply above, and the quote links will take you to that thread if you want to see the complete post.
