Quick answer: Oh. My. God. Get the heavy duty. The regular one is useless and a waste of your money.
Longer answer: The seems like a neat idea, but in order to be strong enough to hold in place with any amount of load it seems that the heavier build is necessary. The light one barely is able to stay in very many different orientations by itself, let alone holding up even something light like a 36" reflector. (I ended up having to also use a spring clamp fixed against another corner to hold mine in place, kind of defeating the purpose.) It isn't just that it's not able to hold much, but that by its construction it seems to be unhappy about many positions and will spring out of them into a straighter configuration — it's not as infinitely malleable as one might imagine.
The heavy version isn't without disadvantages. It is heavy, and it takes two hands to twist it into a new position. But once there, it does a decent job of staying in place. And I've used it with a camera + lens weighing twice the rated load, and a flash + softbox. In either case, I wouldn't expect the steadiness of a good tripod, but good enough for many purposes.
It's not without downsides — it doesn't spring out of where you put it, but because it's so thick it's correspondingly not very flexible. You can't twist it into pretzels — and the best way to make it compact for storage is to straighten it out completely. That means it can't really go in a compact camera bag — you'll need something meant for a tripod or lightstand. And did I mention that it's heavy?
In the future, I'm going to look at the Magic Arm instead. It may be 3× the cost of both flex arms together, and intrinsically less... flexible, but it's rated for 6× the weight without being much heavier in itself, and actually looks to fold up more compactly.