In the US the Mares subscription is $9/month and $70/year. For comparison, the Oceanic+ app is $10 and $80 respectively, with other options available. Neither offer a one-time purchase option though.
The Mares app on the phone looks like it's the exact same as the SSI app and the SSI app sucks. On the phone, the Oceanic app has become more polished and is mostly nice to use with a pleasant UI and cool integration with your photo library. On the watch, the Oceanic app has been rock-solid for over 100 dives (it was less stable when it first came out). Given the "quality" of the SSI app, I don't have high hopes for the quality/reliability of the Mares watch app for at least the next several months (depending on how frequently they update the app).
I've been paying the subscription to Oceanic because I really like the experience of using my watch to log my dives. But subscribing for these apps sucks, and the super high pricing is downright ridiculous. I was really hoping Mares would come in with much lower pricing and force Oceanic to come down in their price. Instead, Mares decided to gouge customers too.