Hi Kvowens,
this is from my limited experience. I've had my uki for about 2 weeks, and loving every second.
Had the pleasure of playing each of the sizes you are asking yesterday.
Size (in order of smallest to largest): Soprano, Concert, Tenor
Went into a music store and played one each of the Kanile'a soprano, concert, and tenor.
The soprano is the "classic" size, with a really bright, punchy sound. Looks really cute, and is so miniature.
The concert is the inbetween size, and the kind I have at the moment. Great bright tone, but I was looking for a deeper tone.
The tenor is the largest of the 3 (not by that much) and the one I played had a deep resonating tone that I just fell in love with. In my opinion the larger you get the more you start to get that "guitar" sound and less like the classic ukulele (other factors like strings/wood besides).
All three I played were incredibly light and portable.