I find it pans out evenly with regards to aftermarket products verses genuine. Some aftermarket products are useless (I remember the faulty heater matrix I had to replace 3 times a few years ago) and yet other aftermarket products are brilliant, wheel bearings, cats, gaskets, alternators etc.
Theres always the worry of the repercussions of using sub standard products for timing chains & cam belts. I think Im going to go with the cheaper kit though.