For me nothing beats a physical book, but when it comes to travel or even just ease of obtaining a book, an e-reader comes in very handy. But which to choose?
Look at the reasons you want one and go from there. Do you want to buy ebooks online? Access books from your library for free? Obtain netgalleys? Integrate with a reading tracker?
When buying ebooks online it’s no surprise Kindle can offer cheaper and wider variety. The Amazon library is huge and they have sales and daily kindle deals. Kobo ebooks can be accessed through the kobo.com.au site and it does have a wide range and some sales, but not as many as Kindle.
For library borrowing most libraries can connect to the Libby or Borrowbox apps. Both of these are available on Kobo. Libby even allows multiple library cards so you get a huge range of ebooks to choose from. Kindle cannot connect to the library.
Netgalley, an online hub where you can request early copies of ebooks in exchange for review, can send direct to Kindle and they recently added Kobo.
In regards to integrating with a reading tracker, Kobo can now link direct to StoryGraph. It offers stats and links to other readers, similar to Good Reads but independently owned.
Kindle cannot link directly to a tracker at this stage, although as both are owned by Amazon I imagine it will link to Good Reads soon.