As far as I know (and please correct me if I'm wrong), the Helix kit reduces the throw but maintains the stock knob height, with zero adjustability. The Craven kit is infinitely adjustable, with a linear relationship between throw and height: as you lower the height, you shorten the throw.
Since you are posting this in the 2nd gen section, I am assuming you are talking about the SSK for the R55/56/57 right? Nevertheless it looks like the exact same designs from the R53, but I'm guessing in different sizes.
They are different designs altogether, the helix just extends the bottom of the existing shift lever by a couple of inches. The Craven speed replaces the complete shifter assembly and features adjustable throws, whereas the helix is not adjustable.
I've had the Helix on my R53 for a coupe of years and really love it, although I had growing pains during the initial installation, such as interference with the heat shield and exhaust resonator from an aftermarket exhaust. After I removed the resonator everything fell into place and works fantastic.
I do get significant heat transfer through the shift well due to the plastic cover that is removed from the bottom of the shifter box (applies to both designs), specifically when the windows are down or the sunroof is open. I've heard of several people that have the same side effect and I believe Nathan found a way to solve it, but I can't remember how.
I had this same issue as well. The fix was to fabricate a new cover for the bottom of the shifter box that clears the new bit installed. The cover is very simple. We created it from sheet aluminum, folded up the edges and screwed it into place. I don't know the exact dimensions. Once in place we used some leftover dynamat from another project to protect the area from heat.
