I thought the real difference was this metabolite of leucine is created in very small amounts within your body, and you'd have to take boatloads of leucine to generate the amount required of leucic acid. It's similar to the 7-oxo DHEA metabolite in PES' Erase, you'd have to take LOTS of 7-oxo to generate a small amount of that 3,7 metabolite.
That being said, I don't know if leucic acid does anything special that would be worth having in higher concentrations. I know what Leucine does, but I'm not so clear on what Leucic Acid does. With more study, perhaps it could be promising, but too early to tell. In cases like this, I err on the side of sticking with what's known (i.e., BCAA).