If you are holding your tongue because you think they may not sound / perform almost identically you don't have 2 of each, because i can tell you they do. There isn't really isn't a difference other than the slightly increased power handling of the 12pi.
If you are holding your tongue because of the Behringer style Development of this product, you somewhat have a point, however there are a few issues that have been addressed with this box that the LAB didn't have going for it. The overall box is bigger (bad mainly) but you can hit lower frequencies with less boxes. Also the increased power handling of this heatsink is a smart idea and i'll give wayne a lot of credit for coming up with it. Also the push pull...well i haven't noticed much of a difference in sound because of it, but hell why not right?
Very very similar boxes and they couple well together in a group of 4 and sounds quite good if you ask me.
Anyways just me $.02
-Josh Billings