Unless you are moving data around constantly, or doing some sort of dual boot configuration,
having a single SSD will offer more storage for the money. A single SSD will be easier to install cleanly as well. (I hate running lots of SATA cables around the case.)
If you require the convenience of multiple drives but only want to buy a single drive,
look into your options for logical disk partitioning. You can make one partition big enough for the OS, then another partition that will store everything other than the OS. That's what I did, and it works wonders for drive cloning and backups, as well as general file system organization. It makes this super easy to just clone the entire partition as a full like for like backup. Like other have mentioned though,
this method does not protect you from drive failure, but it may help with windows reinstall or backup without destroying data on your other partition.
Hope this helps!
-Bahazbz