Whoops!

Typo - I meant Haltech...
With a Haltech (on any engine mind you), you can program your ignition curve however you want over whatever RPM range you want and at what manifold pressure you want. It doesn't have to be a linear curve like the Jaycar kit is.
For example - with the Haltech, you program in ranges of 1000RPM (this is for a haltech E6 by the way - I'm not sure about later models).
In that range of 1000RPM you can program the amount of advance (up to 50deg BTDC!!!) over the full range of manifold pressure/vacuum.
With the Jaycar kit you only have the option of the RPM value when the advance starts, when it finishes and the total advance over this range which then results in a linear (straight line) curve. Addtionally, it only has a vacuum on/off switch to modify the curve according to engine load.
Really, it comes down to flexibiltiy of the programming and how much control over the enigne timing that you want. If you want the works, it comes at a price...
As you said - the Jaycar kit will set you back $120 - a Haltech ignition only, loom, module, etc... would set you back 10 times that.
Confused the crap out of ya yet?

I have to go lie down now...
TC