agree with Oli, and I'll add that you should actually see SMALL performance benefits (relative to RAID-0 on 2 SSD's or on 2 HDD's, which is impossible unless you go out & buy a 3rd disk which matches one of your existing disks) as well as longer lifespan if you do this:
- on the HDD:
** /tmp (approx 300MB unless you download lots of hi-def videos, or else make a "tmpfs" in RAM instead of putting /tmp on the HDD if System Monitor shows that you have lots of RAM being unused even when you're pushing your system harder than you usually do),
** and preferably /var (approx 400MB for typical user) and /Documents (especially if you tend to hit ctrl+S often) -- or even your whole /home (esp if your SSD is small or you delete/add large multimedia files often, have it encrypted, etc)
- put everything else on the SSD
** if SSD is much larger than your operating system (only approx 3~to~10GB of operating system files for most people, and most of the fast SSD's today are huge compared to that), and especially if the SSD is approx same size as the HDD, consider backing-up your /home to the SSD rather than putting /home itself on the SSD: this way when you open a user-file in an app, the SSD will do the heavy work of calling-up the app, while the HDD (slower) will do its part to take a small amount of burden off the SSD by calling-up the user-file from /home.
This way you'll usually be reading from both drives instead of stressing only 1 drive, and putting the smaller burdens on the HDD (except writing burdens will be slightly more on the HDD). This will write (mostly) to your HDD all day and only write minimally to (most of) your SSD's operating-system-files and backups-of-/home. N.B.: IF you backup your large media files often & having a FAST backup is important to you, the HDD's read OR write speed will be the limiting factor compared to MOST ssd's, whether the HDD is being written to (i.e. used as the backup-of-/home) or read from (i.e. is the /home), so a program like rsync that detects which files are already on the back-up-partiton (and thus don't need to be re-transferred) will yield better results than changing whether you use the SSD or HDD as the /home or the backup-of-/home...unless you want to buy another HDD or two, and make them into a RAID-0 and then the 2 or 3 HDD's might BARELY outpace the write-speed of some of today's SSD's that are large but have lower write-speeds than other large SSD's :-) ).
An HDD's speed is fine for the typical (small) office file & of course an HDD streams even a large 1080p file much much faster than you can actually watch the 1080p -- but some people might have rare cases where they'd find it better to put /home on the SSD and back-up /home to the HDD (reverse of the above advice):
* e.g. if having FAST transfers of multimedia files to your mobile device's SSD (or sdxc-card-on-usb3.0, which sadly won't be widely available for approx 6~to~18 months) is important to you, e.g. if you want to replace 10 hours of different 720p videos or massive powerpoint presentations on your mobile-device every morning, then you might prefer /home to be on your SSD instead of the HDD...or alternatively do a backup from the HDD's /home to the SSD every night so the next morning you can transfer any and all files that you choose ON A WHIM THAT MORNING, ultra-fast from the SSD onto sdxc-card, then you won't be late for work. :-)
* etc: infinite possible "abnormal" user-cases.