Hey there, mad_cat.
It really depends on what you plan on using the drive for and your budget. If you don't mind the 50% less storage space of the SSD of the dual drive and it fits your budget, then I'd say - go for WD Black². It will boost your OS performance significantly, plus you get the extra 1TB HDD storage capacity along with it. Note that you'll probably need a mounting kit for your case, as the drive is 2.5", in order to fit it properly.
On the other hand the WD VelociRaptor would be a bit noisier but it has a lot higher performance than the HDD part in the WD Black² dual drive. It's best to use it for high end gaming, 3D rendering, digital image creation/editing and all sorts of demanding software.
Both drives come with a 5-year limited warranty. You can check them out here:
WD VelociRaptor
WD Black² Dual Drive
Hope that helps. Please let me know if you have any other questions.
Boogieman_WD