I have owned an iMac with 256GB SSD and 2TB hard drive. When it failed Apple replaced it with the current model iMac which has Fusion (128GB SSD and 3TB HD). I greatly prefer the Fusion Drive.
When I had the iMac with SSD, I kept my OSX and my Apps on the SSD and I kept my Home Folder on the internal 2TB hard drive. This meant that I never used more than half the space on the SSD. But the biggest problem was that Macs really like Home Folders to be on the Boot Drive. I had large iPhoto, Aperture, iTunes, and Documents libraries that were much larger than the SSD so I needed to put them somewhere else. I decided to put the Home folder (my User Folder) on the HD. This creates a lot of problems. When the computer failed and I had to restore from backups, the stuff on the SSD comes back easily, but the stuff on the Home Folder from the HD had a lot of permissions issues which made it hard to restore from a Time Machine backup. I was finally able to restore most of it after getting through to a highly proficient level 2 expert at Apple Care. When you do put a Home Folder on a different drive, it is very important to put at least one Administrator account on the SSD so that you can boot up if the HD fails.
You can also keep a shell of your Home folder on the SSD and link individual folders such as Aperture, iTunes, etc. on the external disk. This would have made my life easier, but is still fiddly and makes things difficult if you are using iPhoto with iMovie for example.
With the Fusion Drive, everything is simple. You just put everything on the Fusion Drive and let it sort out whether it goes on the SSD or HD. Your home folder is on the same volume as your OSX. When you restore, permissions are restored. Preferences are easily restored. A lot of complexity is taken away. And the performance is very good.
In either option, you need a good backup plan.
In my case, I was backed up to a bootable clone for the SSD, and to Time Machine for both the SSD and Home Folder, and also backed up to the cloud (CrashPlan). This turned out to be important because it turned out the Time Machine was not properly backing up my Parallels virtual machines, and having it in the cloud made it possible to restore.
In my experience, Fusion is the way to go.