Quality; there is not a really big difference, as name brands are generally going to be pretty good because they are in competition.
Blade shape is the real question here.
The reasons for a khukuri's shape have a lot to do with it's weight. Take away the weight, and you start to experience the downsides of the khukuri shape, namely serious throwback from glancing blows. The weight is necessary to drive the lowered belly of the blade into whatever. Without the weight, leverage caused by this dropped blade easily twists the knife in your hand, with potentially injurious results.
I don't think the khukuri shape lends itself well to longer blades of thinner, lighter stock. The Cold Steel part of this conversation should include the point that they are a marketing driven company who caters at their low end to people who don't know better. Some of these designs look cool but may not function all that well due to the design, if not the level of quality.
There is a reason that machete's tips are inline with their handles for the most part, and that is control, which you need more and more of with each added inch of blade length. Especially when dealing with a very flexible and light blade.
The best design for a machete in most parts of north america is the bolo. A latin shape is best suited for tropical regions with extremely large leafy plants where reach is a real benefit and the hook at the end can be used to move stuff out of the way.
You should chose the length of blade considering your height. For example, I'm about 5',3" and my ideal machete length is about the shortest; 18". My brother in law is about 6', 6" and he has no problem swinging around a 24" machete. Physics is part of the customization process when it comes to choosing a tool like a machete. Bigger is not necessarily going to function better for you.