Lower Price or BC? Which is more you important to you? But more importantly, which is more important to the gaming market at large?
Lower price beats a feature that few people would care about. It may seem simple for PS4 to play PS3 games, they both use blu-ray after all, right? But the PS3 is so vastly different than other consoles, while the PS4 is a more standard design. It would take a lot of work to get the PS4 to properly emulate a PS3, and it might not even be possible with their different hardware designs.