For my purposes, I get more utility out of capacity, than small gains in speed. Higher reliability is also questionable, in my opinion, as I rarely have drives fail on their own for me, and I've been using pretty much every brand for years. You might ask yourself what actual components go into a drive in the Black line that are far and above that much different than the other lines. If the answer turns out to be nothing, and yet, the drive performs faster, I would question if the drive is under more stress, resulting in a shorter actual life expectancy. On the other hand, you may find a longer warranty of more value, and if you plan to hammer on the hard drive, you may want one rated for a higher duty cycle than a consumer drive.
I can't really answer these questions for you.