The differences are:
- With a Market auth app you need to keep it installed in the phone, and with a PayPal auth key you need to input it into Meridian main app.
- Market auth app is Google account dependent, where PayPal auth key is device dependent.
- Thus, if you changed your phone with a PayPal auth key, you need to mail me for a new key.
- In the other hand, if you changed your Google account for some reason, you need to rebuy Pro if you're using a Market auth app.
- With price $2.99, PayPal's fee is $0.42 and Market's fee is $0.90. More than twice, but since on PayPal I need to pay effort for device changing request, both are fine for me.
I have also heard PayPal will join Market's payment, maybe it will be the time I totally shift to Market buying.