A 6 string ukulele? If you've ever seen a standard soprano ukulele, you will notice it only has four strings. However, there are three other types of ukuleles, and the number of strings can vary with the type. In the alto and tenor types, you can get a 6 string ukulele. Even with the baritone ukulele, you can get a customized ukulele with even up to eight strings.
More strings add a different quality to the music coming from a ukulele. The strings are paired when tuning, which adds a fuller sound to the chords. Also, some strings can be tuned an octave lower and add more range to the instrument. You will have to learn how to play the ukulele using more strings to get the same chord and this can be too much for a beginner.
Some people take their original four string ukulele and modify it by adding more strings to it. This makes some sense because you will probably want to learn on the four string instrument first, before deciding to upgrade. Instead of buying a whole new ukulele, you can look to modify the original one you have, if it is possible.
The six string ukulele is an upgrade from a four string ukulele. It isn't really for a beginner, and should only be bought after you have some understanding of how to play the ukulele.
You should also be familiar with how to tune a multiple string instrument so that it sounds good when you play it. If you are a professional, this ukulele can give your performance more depth and variety. The four strings variety, however, though simpler to learn is not a beginner's instrument. It is difficult to master, but can make an excellent professional instrument. It all depends on the quality of the sound you are trying to get from your ukulele, and how much money you are willing to pay to get it. That, and whether you are confident enough to use more strings.