Most beginner basses come with round-wounds because these strings are more common (and therefore cheaper, supply and demand or something like that, the kids all want to play rock and roll with twangy sounds).
For jazz/musical theatre I will usually use flat wounds and for anything wanting a little more bite then steel round-wounds. Obviously, as with guitar strings, the tension of the strings makes a huge difference to your sound, as does the material the strings are made from, so you might consider these too. This page is informative, see the paragraphs below the main graphic (I do use D'Addario myself, but other brands are available! Ernie Ball, Martin etc.).
Returning to one particular factor: price. Certainly in the UK (where I am based), flats can be up to twice as expensive as rounds. If you won't be changing strings often and can afford it then go for flats. If you'll be playing a lot the consider low-tension, nickel rounds, which might sound quite smooth but be cheaper than flats. Of course, you might find great deals on the internet somewhere, but I like to go to my local guitar shop.
Fun money-saving tip - you can boil your strings in a saucepan to get rid of dirt and grime and you'll get another couple of months out of the strings.
Finally, let me draw your attention to 'half-rounds', which supposedly give you best of both worlds (flexibility of a round-wound string with the warm sound of flat-wounds). I have never tried these so would be interested to hear anyone's opinion!
To answer the question, if you are a beginner, buy nickel roundwound strings as these are flexible (being round) but warm (being nickel), allowing you to play a variety of sounds. They are also a bit cheaper