Both. I have some ukulele books on Kindle and use my iPad to access them. I can also open them in Kindle on my desktop or laptop computers.
I have our club songbook on the iPad but had to download it to iBooks.
I have quite a few printed song and instructional books. For instruction, I prefer printed on paper. I can mark it up. Plus, have a two-page spread open. Lots of my books are from the 70s. Folk and folk rock stuff. Unavailable now.
For jams and performances, I'll copy a song into MS Word with the chords in place in the lyrics. I can make the text larger and often print it as a two-page spread so the type is large and easy to read. I can write in information about instrumentals and pauses that aren't in the original music.
I put the music I'm going to play in a binder in the order of play or alphabetical if the set list order isn't established. Sometimes I'll write in a short introduction about the song.
At the uke club, when we were still meeting, we projected the song on a screen so members wouldn't have to bring music and a stand. The downside is that they also can't make notes about the songs. And the way it's supposed to be played. Which isn't always the way it's written. (One of my pet peeves. Write it out exactly the way it's supposed to be played and sung. Be consistent in the symbols, abbreviations, notations, strum patterns, etc. Otherwise, people get very confused.)
So, both actually.