One of the ways to look at this is to start off by eliminating what won't work.
The smaller 15" models such as the BAS and 15 Ang Khola are easier to carry but would require more effort with prolonged use. I'm not saying they won't work but you can think of them like a small folding camp hatchet, they can get the job done in a pinch but not what you would take for a day of hard work.
The 20" Ganga Ram and Super CAK are (in my opinion) really novelties and are much too large for most people. I have a large Ganga Ram and have yet to find anyone that can use it. The handles are as large or larger than most ax handles and cause lots of hand fatigue and they never feel comfortable or secure. These models are also very very heavy. Again you can think of them as carrying a full sized ax to prune your rose bush.
That leaves the 16.5 - 18" models. They are big enough to do most jobs and yet light enough to make carrying them a realistic possibility. The most popular models in this class are the ASTK, Bonecutter, Ganga Ram & CAK. The M43 is thought of as a jack of all trades but even though this is my favorite model in its current configuration it is not the tool it once was but, it is still a consideration.
There may be some other models that are this same size that may even look alike but the models listed above are what the kamis (blacksmiths) consider working models as opposed to others that everyone assumes will live out their lives on someones desk. This is important because you have to remember that a human being is making these tools and on some models more attention is given to carving the pretty handle than worrying about the hardness of the blade. Likewise, on models that are considered "tools" less attention is given to "pretty" and much more goes to balance, feel and construction.
Now to be fully honest, a small forest ax would be as good or better in many situations as any kukri. That being said, most of the people on this forum just prefer to use a kukri just like some people prefer to hunt deer with less efficient bow when the rifle is available.
***When you order your kukri be sure to tell them that you want it for work and you want a village (satin) finish on the blade rather than the polished blade. In carbon steel, the polish will be ruined quickly leaving you with a half shined, half satin finsh that looks nasty. The polishing also softens the carbon steel making the edge harder to manage, especially in the beginning. Lasty, I wouldn't get all hung up on the full tang issue, for centries the vast majority of the models have been made with a stick tang and they obviously work. I'm not saying that you shoudn't get one with a full tang, just don't get hung up on it. My favorite HI kukri (by far) is the Bonecutter, which is a full tang but in this case it works and is very nicely balanced.
Good luck on your search!
Edit: Here is a perfect example of a kukri NOT working. This is the product of adding a full tang to a design made for a stick tang, which changes the balance to the handle and takes away from the chopping ability of this kukri. The edge bevel is also partly to blame here, its design literally stops the kukri from penetrating more than an 1/8 of an inch on each strike. A Bonecutter (or just about any of the 18" models mentioned above) would chop this branch in less than half the strokes. If you are familiar with good working models you can close your eyes and hear that this kukri is not working and feel the vibration in the hand. (This is obviously not an HI model, the HI BAS will be properly balanced and chop much better than this one)
http://www.youtube.com/watch?v=yC2dGZD5ngA
(***Karda, if adding this link is an issue I will remove this edit. I am not trying to bad mouth any other company, just pointing out the design issues that make or don't make a good chopper)
