ATR headers = stainless, won't fit angle plug heads (Bowtie, Brodix). Locates the turbo in the stock location, so you can use stock downpipe, or ATR 3" downpipe, or PTE 3" downpipe
KB headers = mild steel, will fit angle plug heads. Only downpipe that will fit is the KB 2.5" downpipe that comes with the header kit. No other downpipe will bolt up, unless you do a custom one. There is a provision for an external gate on the crossover as well.
My vote is the ATR headers, since they are stainless, and any downpipe will fit them, and I'm especially a fan of the PTE setup.