From my short lived (about 2 months) experience with GPLC, it has been absolutely fantastic - seriously eye opening. It has been such an effective supplement, as my supply dwindles, I am actually hoarding away the remaining dosages for certain muscle groups, because it is truly THAT good. It is so rare that I can feel a supplement work, I have never really taken price into consideration as a drawback.
I would LOVE to know how the other iterations and bonds of Carnitine perform and exhibit their specific set of actions vs. GPLC; such as the prop version you mentioned above, as well as the L-Car-L-Tar and even the Acetyl type. Does each newer and more stable version simply climb up the rankings in efficacy with a linear pattern, concerning androgen receptor modulation, endurance and fat burning support, blood flow augmentation, vascularity and muscle load capacity, etc? I would find this to be nearly impossible, and would tend to think each version has its own strengths - where as GPLC has the demonstrated affinity to exert its actions within muscle tissue (which is why it might be a superior choice in the realm of bodybuilding specific intent and usage)
Along with the above information and comparative analysis between two or all of the Carnitine varieties... I would also like to try a gram for gram identical dosing protocol pre-workout to come to a better conclusion.