The Six-String model is available in Tenor and Baritone. These sizes are differentiated by body size, scale length (the distance from the nut to the saddle--this distance determines the spacing between the frets), and tuning.
The baritone is a larger body than the tenor, which is needed in order to accurately represent the lower notes of the baritone tuning. The baritone is tuned d-g-b-e (some think of this as guitar tuning), while the tenor is standard ukulele tuning (g-c-e-a). In both cases, the 1st string and the 3rd are strings are doubled--typically octave-doubled, though some people choose to string them in unison (either the higher or the lower octave).
The tenor is a 17" scale length, while the baritone is 20". This gives the baritone more fret spacing, but does make some chord stretches more difficult.
When choosing your custom ukulele, you should take into consideration acoustics, comfort (how the body size feels), and playbility (primarily affected by fret spacing). There is no right or wrong answer.
You can visit our sound sample page to hear different woods in combination with different sizes of ukuleles.