The AKG K391-NC are a good set of in-ear buds that sound great and isolate the listener really well. The outstanding noise cancellation and leakage performance are recommended for noisy commutes. The sound profile works well for most music genres but their in-ear design might not be for everyone.
The K391 have a simple in-ear design that may not be as comfortable for everyone. They seem durable, and the control module is lightweight and portable. However, the cable is a little thin and does not provide adequate in-line controls apart from Play/Stop button.
They are as comfortable as you would expect in-ear earbuds to be. The sleeves come in 3 sizes, so unless your ear canals are unusually narrow or wide, you should be able to get a comfortable fit.
The buttons are in the right place, and the Control Module is small enough to fit any pocket. The in-line remote has a mic and call/music control. No volume up/down.
The AKG are decently stable headphones. Their in-ear design fits deeply into the ear canal making it hard for them to fall out during moderate physical activity. However, They won't be sufficiently stable for high-intensity exercises and the control module, and long cable can easily get hooked on something and pull the headphone out of your ears.
The AKG 491 will are highly portable headphones. The in-ear design is compact and doesn't take too much space which makes it easy to transport in a bag or purse. The control module adds a little more bulk than basic passive in-ear headphones but does not reduce their portability as they will still comfortably fit into most pant or jacket pockets.
The K391s excel at isolation. The "in ear" design provides passive isolation and a tight seal that leaks very little sound. The active noise cancellation also does a great job with reducing ambient noise. This isolation performance is ideal for noisy commutes and international travel, provided you don't mind the "in ear" design.
These headphones have excellent noise isolation, especially at higher frequencies. passive noise isolation is very good down to 500Hz. Below that ANC kicks in, which maintains the excellent isolation performance. The only point where they seem to struggle is around 400Hz.
Very good leakage performance. They leak more around 4KHz, which means higher notes will escape.
The K391-NC only have noise canceling as an active feature. This makes them less versatile for other use cases other than noise canceling headphones, as they lack both wireless or an app, to enhance and customize their sound. On the upside, they can deliver a good battery life that lasts more than 35 hours of continuous playback. Unfortunately, they also take an exceedingly long amount of time to recharge.
The AKG 491-NC have a long battery life but take considerably longer than average to charge. This may be because they use an older mini-USB charging port. Unfortunately, this means you have to charge your headphones overnight which is not always practical. On the upside, they deliver above a day worth's of battery life, and you can use them while charging so they are decent headphones to use at the office or if you have access to a power source while on the bus, train or plane.
No compatible app.