Just saying it makes very little sense to go fastest here.
Go highest density and get a 1TB 5400rpm one.
You will likely only put backups, movies, music, images and well big files on there. Thus all that matters is sequential speed and the a 5400rpm of the highes density is so little slower than a 7200rpm 750GB WD Black that you would never notice the difference. Not if you had just two of otherwise the same notebooks with those data drives if front of you and did normal stuff. Without actual benchmarking you'd have difficulty figuring out which is which. Considering how much slower HDDs get on the inner tracks after a certain amount of the data saved on it it is better to setup your partitions intelligently rather than go for the fastest drive possible. If you need faster speed for some stuff than make a partition at the beginning of the drive and save unimportant backups and stuff on a partition at the end of the drive.
The fastest possible which would be a WD Black or a Momentus XT just makes no sense in that setup. Those disks are reasonable if they are used as system disks, or for higher IO workloads not for the stuff people use it if they already have a 256GB SSD.
You'd just have a more noise HDD that sucks more power and is expensive for really nothing in return. Sorry it is just stupid to pair a WD black with an SSD.