I waffled between a 2.5 SATA SSD and this for a while when considering upgrading my machines.
I briefly considered upping the game on Velociraptors, but eventually I came to the simple conclusion that a PCI-Express SSD is currently the best upgrade you can do for your computer. Unless you're crunching massive files of one kind or another, more processing power in your CPU, video card, or RAM isn't going to help you see an increase in speed.
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Except one small thing - SATA still bottlenecks speeds.
I know, the concept of "bottleneck" when it comes to how much data can get shoved through the pipe is vastly beyond anything from two years ago, but the speed of data transfer is still the key ingredient.
Hard drives don't often have a direct connection to the motherboard, but that's why a PCI-Express card is a brilliant form factor.
Considering that I've got my seven SATA slots maxed with DVD drives and hard drives, adding another hard drive was going to require upgrading one drive over another.
The issue really boils down to interface and speed.