Life can be funny. I just splurged about $300 on a Kobo Forma, but lately have been reading more books on my phones, the Samsung Galaxy Note 9 and the Google Pixel 2 XL. Now, I'm debating whether to return the ereader. because I'm not getting the eyestrain I expected from an emissive display.
Books surrounded me in childhood so there will always be a special place in my heart for them. However, from 2000-2006 I mainly read on my PDAs with the Microsoft Reader app. Yes, the screens were small, however the convenience and portability were a game changer. Then came the Kindle Keyboards and e-ink, and I thought ereaders were optimal since the last decade.
Now?
Google Plays Books + 6.4-inch OLED screen with >500 PPI is very nice.
Google's reading app makes syncing immediate so I can resume precisely on my desktop, laptop, tablet, work phone, or personal phone. Notifications can be removed so I'm not interrupted ever. Not to mention, I can use Twilight (similar to f.lux) or the phone's built-in night mode to change the color temperature and brightness of the screen even more than Kobo's hardware allows. I can make the background sepia or 'white text on black' immediately because a 2018 phone's processor is so powerful. Merriweather or Literata are two superb fonts on Google's app: they are more readable at dimmer brightness than Bookerly or Malabar on a Paperwhite 3 or Forma. The much higher PPI means text is so sharp. Also, the phone's volume buttons can be used as page buttons on the app.
The only advantage I can think for the Kobo Forma is the possible eye relief. I say possible because I can't find any recent studies comparing e-ink to the modern OLED/LCDs on phones (with adjustable night-mode). Also, if you read graphic novels/mangas or PDFs, then ~6-inch phones are still too small.