1) SSDs are closer to 40+ X faster than a HDD, When looking at a OS + program drive. HDD typical access time is around 12->12.6 millisec while a SSD is about a tenth of a millisec. This results in a BIG diff on the important 4K random perfoformance
2) Sata III HDDs is a marketing tool - Who wants to market a sata II. They are just saying it works on sata III. The ONLY benifit of a sata III HDD on SATA III port is in the SHORT lived burst speed. HDDs can not saturate a sata II inface for sustained throughput.
That said: SSD will considerably speed up boot time and program loads and reading/writing files on the SSD. They DO NOT increase how fast a program runs, does not speed up internent broosing, nor email not downloads nor FPS in game play.
The vast majority that use an SSD for a OS + program drive would NEVER go back to a mechanical HDD.
But have to way cost as an SSD is just not cost effective to store a ton of Photos (jpeg/bitmap files) nor Movies. So you normally buy an SSD just for your OS + programs and a FEW games, then store all of your generated files on a HDD.
Added: Then there are the Hybred drives which combines a Small SSD with a Mechanical HDD - For some this is a good option, just wish they would go with a larger SSD
Also not just about speed, I find them more reliable, they normally draw less power. Smaller packaging is because they DO NOT need the volume of a 3.5" packaging and they only need +5 V where as 3.5" HDDs also require +12V
See:
http://www.storagereview.com/ssd_vs_hdd
PS older review as prices for SSD have come down (just under a buck per Gig)
A little more on hybred drives:
http://www.pcworld.com/article/2012223/hybrid-hard-drives-how-they-work-and-why-they-matter.html