There is no universal answer that applies to everyone. Some basics:
- it's possible for a 5400 RPM drive to have better performance than a 7200 RPM drive. While the 7200 RPM drive will always have the better access time, data density on the platters determines thruput performance.
- flash on an HDD like the Seagate SSHD stores the recently used files in flash memory so it is retained in the flash if computer is put to sleep or hibernate. In other words, faster wake-up time
- cache on an HDD retains the most recently used information read from a disk, and written to a disk; also enhances write performance as the computer/OS doesn't need to wait for a write cycle to complete bebefore continuing with other i/o tasks.
- a cache on an HDD will sometimes prefetch information along with the specific i/o read request. Some disks will read in the entire track to have it available in the cache just in case a file is stored sequentially on the disk, and the OS is reading only a block at a time.
- an SSD definitely is better for random I/O and sustained reading/writing, but it comes at a premium price.
- a fusion drive tempts the user with better performance via a compromise; most recently and frequently used information is migrated to the faster SSD storage pool, while the less frequently used blocks reside on the slower HDD. It is not redundant, as the data resides in one place or the other, and I believe a file does not straddle both. there is overhead within the OS that migrates files to the best place within the storage pool dependent on its use.
Many will look at the question based on $$$ and utility. Each of you need to evaluate your true needs. None of you will be spending the majority of your time running benchmarks everyday, so you actually need to consider how your are using your computer most of the time.
As for the DIY Fusion drives, be prepared to watch over your system any time Apple releases a system upgrade, and make sure you have redundant backups in case anything goes amiss. It is a great technology, but Apple won't be watching your back if you have a DIY solution. Be prepared to understand the technical details and the tradeoffs.
A faster SSD or HDD or SSHD is usually a welcome improvement, but your budget will determine what you can afford, and your actual daily usage will determine what may actually benefit you.
I've got several laptops, PCs and Macs; some pure HDD (7200 and 5400 RPM), my primary laptop has a Momentus XT 750GB SSHD, and several with built-in or upgraded SSD. To me, the SSHD upgrade helped my workhorse laptop, and performance improvement was immediately recognizable. The laptop I upgraded from HDD to SSD also was a day and night difference, with the SSD clearly a big improvement.
With prices falling every month with SSDs, and both reliability and performance a clear improvement, I'm a holdout for upgrading everything to SSD, as I've only needed to update a few of my computers.
Whatever you decide, things will be even better next year!