Mag Hitec has been around a lot longer, but that doesn't really say anything about ATS. Mag doesn't make manifolds, so if you really want to stick with one brand ATS would be the only choice.
I glanced at the ATS covers and the only thing that made me raise an eyebrow was the "heavy duty cast material". Okay, what material? I'm assuming aluminum.
Also note that ATS doesn't make a front diff cover, but Mag does.
Ryan