Speed: SSDs shine when it comes to speed. An SSD-equipped PC will boot in less than a minute, and often in just seconds. A hard drive requires time to speed up to operating specs, and will continue to be slower than an SSD during normal use. A PC or Mac with an SSD boots faster, launches and runs apps faster, and transfers files faster. Whether you're using your website for business, school, or fun, the extra speed may be the difference between finishing on time and failing.

Fragmentation: Because of their rotary recording surfaces, hard drives work best with larger files that are laid down in contiguous blocks. That way, the drive head can start and end its read in one continuous motion. When hard drives start to fill up, large files can become scattered around the disk platter, causing the drive to suffer from what's called fragmentation. While read/write algorithms have improved to the point that the effect is minimized, hard drives can still become fragmented. SSDs can't, however, because the lack of a physical read head means data can be stored anywhere. Thus, SSDs are inherently faster.

Durability: An SSD has no moving parts. Most hard drives park their read/write heads when the system is off, but they are flying over the drive platter at a distance of a few nanometers when they are in operation. Besides, even parking brakes have limits.

Availability: Hard drives are more plentiful in budget and older systems, but SSDs are becoming more prevalent. That said, the product lists from Western Digital, Toshiba, Seagate, Samsung, and Hitachi are still skewed in favor of hard drive models over SSDs. For PCs and Mac desktops, internal hard drives won't be going away completely, at least for the next couple of years. SSD model lines are growing in number: Witness the number of thin laptops with 256 to 512GB SSDs installed in place of hard drives.

Form Factors: Because hard drives rely on spinning platters, there is a limit to how small they can be manufactured. There was an initiative to make smaller 1.8-inch spinning hard drives, but that's stalled at about 320GB, since the tablet and smartphone manufacturers have settled on flash memory for their primary storage. SSDs have no such limitation, so they can continue to shrink as time goes on. SSDs are available in 2.5-inch laptop drive-sized boxes, but that's only for convenience. As laptops continue to become slimmer and tablets take over as primary platforms for Web surfing, you'll start to see the adoption of SSDs skyrocket.

Noise: Even the quietest hard drive will emit a bit of noise when it is in use from the drive spinning or the read arm moving back and forth. Faster hard drives will make more noise than those that are slower. SSDs make virtually no noise at all, since they're non-mechanical.

Overall: Hard drives win on price, capacity, and availability. SSDs work best if speed, ruggedness, form factor, noise, or fragmentation (technically part of speed) are important factors to you. If it weren't for the price and capacity issues, SSDs would be the hands-down winner.