When you're looking at string instruments and have narrowed it down to the guitar vs ukulele one factor that you're going to look at is which is more difficult to play? The answer is that the ukulele is a great deal easier, and is a great choice for a number of other reasons.
The guitar has a lot more strings than the ukulele, making it easy to see why it's a more difficult choice. It's also a lot bigger, a lot more popular, and much more of a common instrument. This means that it will be harder to carry around, that it will be easier to find friends who also play it, and people will be more accustomed to learning you play the instrument.
The ukulele, or uke, is a much smaller instrument, making it easier to learn how to play and to carry around. It's also a lot less common than the guitar, which can be seen as either a bad or a good thing depending on your viewpoint. It can be a lot more fun to pull out a uke and surprise your friends. They may laugh at first to see you playing such a small instrument, but once you start playing the instrument they'll quickly be impressed with the songs you can play--and it won't take you as long to learn how to play them.
Of course, as with any instrument, the more you play the better you'll sound. You'll probably start off learning quick and simple songs, like jingle bells and happy birthday, but eventually you'll build up to playing uke versions of popular songs. For some ukulele inspiration check out the likes of Jake Shimabukuro, Jack Johnson, Ingrid Michaelson, or Amanda Palmer for some great ukulele playing.
The ukulele is a great instrument that is fun to play, comparably easy to learn, inexpensive, and an impressive thing to pull out at parties.