I would propably get the cheaper one between RE3 and Black with the same size, but if Carpetsmoker says that RE3 got better reliability, then I am buying it
Also RE3/Black are created with about ~340GB platters (all from these series), so performance of 0.5TB and 1TB should be similar. The only 500GB per platter current WD drives are 2TB Green ones, with 4 x 500GB platter and several energy efficency technologies (like varying speed rotation of plates between 5400 and 7200 rpm), but they are suited better for storage, not for system disk .