hotshot comes in either an older 4-1 design and a newer 4-2-1 design (youre likely only going to find the 4-2-1)
stillen comes in a 4-1 design
obx is hard for fitment and are almost all 4-1 design.
the differences between the 4-2-1 and the 4-1 is next to none in power... i wouldnt worry about smog or anything with headers as you wont pass with either one if im not mistaken? it depends on your emission laws