One-Time option for Pro
$40 a year seems a high price to pay for an app. I feel that you should be able to pay a one time fee (that is significantly less than $40) to be able to upgrade to pro.
the problem with the subscription model is that as consumers we have too many subscription services we don't want another draining money every month. You could do an in app purchase for major updates (not security patches). I'd be more willing to pay for a major update than another monthly subscription. And don't do like EA does and forgive a stripped down featureless app and after we buy the app charge us extra to buy basic functionality. You could even separate the app into different functionalities and let people choose which functions are useful to them through plugin purchases. Like right now I don't have a PC so I have no use for that functionality. The only thing I want out of this app is pushing to my Nexus 7. It would also be awesome if it pushed the notifications in a way that a user could use his messaging app of choice. I would rather use Google messenger over the mightytext app.