As someone who is familiar with Windows 8 SS (such a bad choice of names, with that acronym) and FlexRAID (and has used DriveBender, too!), they really address different groups.
FlexRAID is for people who would have a huge shiny enterprise level RAID solution, but can't afford it or can't justify the cost. It's not that easy to set up, but it does a fine job and is largely set and forget. The parity and mixed-drive-size options are robust.
Windows 8 Storage Spaces is for people who can't remember if the new Bieber album they downloaded is on D: or E:, and have no backups nor any idea what parity is.
Both do a job well, there is some overlap, but the focus is very different.
Razorback - Arch-6430U - i7-3687U - 16GB - Intel HD4000
Rocinante - Ubuntu-E5530 - i5-3230M - 16GB - Intel HD4000
Donnager - Arch-M4800 - i7-4800MQ - 32GB - Intel HD4600/Quadro K2100M
Vargr - Arch-Vargr - i7-4790K - 32GB - Intel HD4600/GeForce GTX 970