I would get the SSD, you'll have more benefit with loading applications and booting. Physical media is well suited towards storing larger sets of files. Stuff I use a lot is on my SSD raid-0, stuff I don't use as much goes on my RAID-5.
Mine is like this:
Windows, SSD.
Civ 5, SSD.
StarCraft, RAID-5.
VMWare Workstation, SSD.
VMWare virtual machines, RAID-5.
Video, Music, and Pictures, RAID-5.
Web Browsers, SSD.
etc.
The basic theme is where most commonly used applications and files get put on SSD, files and applications where 1gb/s won't make a difference or amount of storage is more important, use the HDD (in my case a RAID-5 of 3x1Tb drives.) For things like video, as long as it can be read quick enough to be viewed smoothly, it doesn't matter that it resides on the RAID.
Also a little info about RAID-5 for those who don't know what it is. It is data stripping with a parity "drive" (really a parity block every n blocks in the RAID where n is the number of drives.) Think of RAID-0, but you have an extra drive so every n blocks, you store information about the last two blocks. So if any drive fails, the parity blocks can reconstruct any data from either of the two other drives, but not both at once. (3-drives have 1-drive tolerance for failure, 2 drives of the 3 must fail to lose all the data.) Also, when a drive fails your RAID-5 is still accessible, but it runs "degraded," since it is a drive short.
So SSD RAID-0 backs up to the RAID-5, and the RAID-5 backs up to an external drive. I used to boot off of RAID-5, but over time it slows down quite a bit. The secondary benefit of RAID-5 is that read speeds are improved because you're reading like RAID-0 across 3 drives. Conversely, write speeds are hindered because you need to write 33% more data (with 3 drives,) just to write the extra parity block... but believe me, it's worth the redundancy... but that is slightly off topic.