I suspect most people have about 100GB of OS and Apps (which will obviously live on their SSD) and then some mix of the following:
Documents (Office/iWork stuff)- this is probably a minor consumer of space and can live anywhere since performance is not all that important... SSD, external, NAS it probably doesn't matter for most. If you don't have massive amounts of this, just keep it on your SSD.
Media (iTunes, Movies, etc.) - this is probably a huge consumer of space and doesn't need the fastest storage solution. In a multi-computer household/office a NAS is probably best, if your nMP is the only computer that consumes this content, then external USB3 is probably the most cost effective.
Project files (Photo Libraries, FCPX Projects, Logic X Projects) - I would further break this down into current projects and archived projects. To get the most benefit from SSD performance, I would ensure you have enough SSD capacity to at least house your current project/library. Obviously the size and frequency of your projects will dictate the size of SSD you need in order to not be juggling files so often it becomes a PITA.
Archived Projects - Move older archived projects and libraries to spinning disks (TB or USB3). How fast you want or need this storage to be, will be somewhat dependent on how often you access it, and how patient you are. On one extreme, you can go for an expensive TB enclosure with a bunch of drives in RAID0, or on the other hand, you can just use a cheap USB3 SATA HD dock and put your archived drives in a drawer until you need to access something. I purchased a WD Velociraptor Duo which offers a nice mix of capacity, price and performance (2TB for $500 that has good random I/O and 350MB/s sequential I/O). I'll use this to house my last couple archived Aperture Libraries. So effectively, my current Aperture library is on the nMP SSD, my last two prior to that will be on the 2TB WD external, and anything older than that is stored on my NAS and an offsite backup (more on that below).
Collections (of clip art, stock photos, videos, audio samples) - Like media, this can be huge consumer of space but speed can be a factor as you're often using this stuff in your project work. In an ideal world, this stuff would all live on solid state storage with super fast response times, but if that's not economical, then putting it on a RAID0 array attached via TB is probably the next best thing. Again, this is where the WD Velociraptor Duo is a nice product (I have no affiliation, I just like this product now that the price is reasonable).
Backups - I'm one of those that doesn't believe RAID is a suitable backup. RAID can be a suitable solution for guarding against drive failure with data you don't need to backup such as media/collections that can obtained again in the event of a loss. However, the problem with RAID1 or Parity RAID, is that it adds complexity, cost, and can't protect you from corrupted data or dumb user mistakes. There's nothing worse than having a perfect/redundant backup of a corrupted file. Personally, I really like Time Machine because it works transparently in the background without me needing to do anything and stores multiple versions. So, my backup strategy is multi-fold... My main SSD (with OS/Apps/Projects) is backed up with TM to a Time Capsule. When I archive a photo library with my new setup, it will get copied to three places... the WD external attached to my nMP, a drive on my NAS, and a large USB3 drive I keep as an offsite backup (when the NAS or offsite drive gets full, I pull it, put it in a drawer and start a new drive). I don't backup my main media library as I can recover that if I need to by downloading it again.
