My 2c...
when safety is involved, always buy the genuine article. It may be more expensive, but may very well save your life.
eg. brakes - I will always upgrade the brakes as one of the first things on a car now, after historically upping the power beyond the scope of my brakes and running into a spot of trouble. I will always spend as much as I can on brakes.
Conversely, a copy body kit (not that I would buy a body kit, I like factory stuff but that's an aside) may well look and do the job of a genuine kit (pending that you don't have to cut / remove the front bumper support etc), so in that case if the saving is substantial, then I'd say give it a go.
I wouldn't take the gamble with a seat - what happens if, in the case you rear end someone the seat back collapses forward? or the seat frame rips out of the rails? not what you want...