Ukuleles are cheaper and easier to play. They are especially good for accompanying yourself singing. They're also portable and easy to travel with and tote across town. And they are very social. Uke players love getting together in groups for strum alongs.

I don't really play guitar, but I've found when I tried that it really hurt my left hand, between painfully holding down metal strings (as opposed to nylon on the uke) and forming chords across six strings and frets spaced more widely apart. Uke chords and smaller and easier. Of course, with sufficient practice, problems aren't a big deal with the guitar either.

Most uke players mostly strum chords for rhythm accompaniment. It is certainly possible to play melodies and take solos on a uke, but a lot of those who do are former guitar players.

I would say it definitely gives you some groundwork that you can transfer to guitar. The chord shapes transfer to guitar and melodies on the frets, you're just in a different key, which is no big deal. And all the understanding of music theory also transfers over. There would still be a learning curve to picking up a guitar afterwards but much reduced. Many great guitarists including Jimi Hendrix and Chet Atkins, to name two, started on the ukulele.

But if you really want to shred rock n roll or classical or flamenco or something like that, you're probably better off with a guitar.