SSD's and normal 7200 RPM and 10000 RPM drives:
The greatest differences between SSD's and what we would call "normal SATA III drives" (henceforth referred to simply as "HDD") is price per gigabyte and read/write speeds per drive.
SSD: Higher price per gigabyte, but faster read/write speed.
HDD: Lower price per gigabyte, slower read/write speed.
Both of these gain from being in any RAID configuration. Mainly for gaming RAID 0 is used, as it increases read and write speeds, but at the cost of reliability.
The issue with HDD's in RAID 0 is that you need several drives to gain up to just one SSD in terms of read and write speeds and you lose reliability. If one of your drives crashes/breaks, all data is lost.
The average R/W speed for a 7200 RPM HDD is about 70-100 MB/s, for 10,000 RPM drives we get up to around 100-130 MB/s. While most SSD's used in gaming setups easily go up to 450-500 MB/s. (These numbers are rough and do not always represent actual usage speeds. Values may vary on drive age and manufacturer/drivers)
So, if you want more space and pay less and aren't worried about a RAID 0 configuration messing things up, go for a few HDD's.
You want to ensure you have constant high R/W but don't mind having a bit less space available, go for SSD's.
Today SSD's are cheap enough to be used for pretty much any storage (with perhaps the exception of media storage) and offer greater speeds than commonly needed, even in gaming.