For Android, all the "native" Ampache apps are crap. Development has been dead for years it seems. For Android Subsonic apps, the only decent free one is Utrasonic (works even better that the official free Subsonic mobile app), but even that one has its limitations/bugs and development has been dead for a long time. I went ahead and bought DSub and it really IS the best out there. It is actively developed and many compatibility issues between Ampache's Subsonic API and it have been ironed out. I don't just recommend it for Android, I would be bold enough to say it's the only one worth looking at for Android and worth the money.
I don't have an iPhone so, I don't really have a say in that area.
If there was actually a native Ampache app that took advantage of all of Ampache's features (like Radio Stations or mobile control of Channels), then I could probably make a realistic comparison. But, unfortunately there isn't any (or I'm just not aware of and never found one).
The Subsonic API has features that Ampache does not have anyway (like Podcast syncing, Bookmarking, and Chat). So, you're really not missing out on anything using Subsonic API. And if Ampache adds those features, it's only to fill in the gap.
That's just my opinion as a normal user. So, anyone else with more insight into this I would be very interested to hear, too.