I was wondering the difference because I'm probably going to get an Alienware soon and one of the options for a hard drive was a 1TB 5,400 RPM + 64GB mSata caching drive for $225, then there was a 500GB 7,200 RPM + 128GB mSata boot drive for $175, and I was wondering if there was a better one out of those two. I heard that the caching loads files faster but takes longer to start up, and the boot starts faster but loads files slower, and I was wondering which one is better or worth spending more or less money, and it's all a little confusing.
The cache drive doesn't actually speed up your system right off the bat. It compiles a list of commonly accessed files and moves them to the SSD, so that over time the most commonly used apps / files are accessed very quickly.
The other option actually loads the OS onto a SSD, so all OS files and any apps installed on the SSD are accessed quickly, while the HDD is used for storage or larger files that are not needed all the time.
Basically the cache drive is kind of a hybrid. It combines a HDD and SSD, and speeds up some things. While the boot drive is similar to building a computer with a SSD and a HDD. You install the OS on the SSD and use the HDD for everything else.