Paper and digital guides are of different quality, and you want the best one, independently whether it is paper or digital. The best bird book for Sweden is Collins Field Guide by Svensson and others, 3rd edition is coming soon. It exists both in paper and as an app. For other regions of the world, you an ask on Birdforum. There is usually a good consensus what is the best one.
If your phone is reasonably good, you can easily cheat: take photos of all pages of your book. Don't use any app which must be online to function, because internet coverage is unreliable in birding places, even in most civilized countries.
And as others told you – going out for a day near home, a phone is easier to carry. If you go for a longer trip, for example a dream travel abroad, it is foolish not to take a paper book in case your phone fails.