I did have a Kobo Aura but sold it on. The Aura is a nice device with a flush bezel, and it's cheap, but there's a reason for that, the screen is not as clear as your other choices. It's not just the PPI, it's more a murky layer over the font layer which makes the screen not as sharp as it could be. It's okay, and probably similar to your existing NST (yes I have one of those too
) but it's no where in the same league as the newer devices.
There's a reason Kobo has discontinued the device.