The SSD system will be way faster then the hard drive solution - and more reliable (SSD is and order of magnitude more reliable then hard drives) - you will be paying more for storage however.
Its impossible to advise the difference unless the load (and specific disks) are known, but for a typical use case I'd expect the SSD's to be in the order of magnitude of 2-3 times the speed of hard drives if managing fewer larger files, and 10 times+ the speed for lots of small files scattered throughout the disk.
If you are looking at something to act like a NAS, considure a combination of HDD's and SSD's and ZFS to get the best of both worlds.
The "sequential" speed of an SSD is in the order of magnitude of 5 times that of a hard drive (and this figure would, in some cases be reduced because you have 3 times as many disks on the hard drive), however the magic is the 100x order of magnitude difference in scattered reads - because hard drives need to wait for the platter to spin under the head, while there is no mechanical delay accessing an SSD.