Ipad app sound level meters vs a portable sound level meters

From the bit of investigation I’ve done the Apple stuff does a pretty descent job on its own. If you’re looking for that natts Ass accuracy I would say buy a meter. 

I find decibel app on iPhone works well to help get a handle on the sound in one’s room.

You might want to get a calibrated mic. rather than rely on the built-in mike. I found the ipad very useful to set up EQ for my car system.

I can't speak for the Apples Sauce AP, but for the Android software I have tried and trust, UNLESS you are trying to use it as evidence in a court of law, the Android Ap is quite good and representative of what you are hearing. I actually enjoy watching the visual representation of my music and sometimes even learn something about the music from it. I use two different APs,
DecibelX, which gives the most information and

Sound Meter, which is just a basic SPL device and has the capability of being calibrated.

Dayton makes a tiny calibrated mic if your device has a jack for it.

https://amzn.to/43p6AEE