On Backountry, they only have the PX 12 Jib Micro Heel so I'm assuming it's just the new model, maybe they're getting rid of the regular PX 12? It mentions a shorter mounting zone, so that could be where they get the micro, but it according to Backountry it weighs the same as the PX 12 Jib.
If you liked the PX 12, I don't see a reason not to get the Micro Heels.