They require the slide to have a different rear sight cut. Hajo requires the Shadow cut, and LPA requires the LPA cut.
There's probably some other differences but that's all I see.
-Jason
I've tried both, and like LPA better for sight picture -- closer to slide/bore - LPA seems like its part of the slide, while HAJO riding up top taller. Also the notch is narrower, while HAJO notch wider like the fixed competition rear sight, but if you don't like it that narrow, I imagine you can file it wider.
I ordered the Accu-Shadow with the LPA option and been VERY happy with it.