I was wondering which will be a better choice in the long run?
A 64GB SSD for caching or a 128GB boot drive?
Caching will allow me to get a tangible performance boost on a huge number of applications as well as games at a time whereas boot drive will give me give me supercharged performance on a couple of applications and games.
The 128GB boot drive gives not only the advantage of not needing to wait for data on it to be cached, but you can also allocate some of its space for caching if desired. The 64GB cache-only drive option excludes this dual-functionality. If the drivers are pretty much equal in every way besides intent and capacity, I'd go for the 128GB drive.
If 64GB is only enough for you to use as caching, but 128GB can be used for a boot drive, then you can also use some of the 128GB drive for caching like you would have done with the 64GB drive. For example, lets say that you only need 64GB of the 128GB drive for boot drive purposes. The other 64GB of it can be used as a cache (or any other capacity of it that you deem better if you want to), giving you the best of both the boot drive and cache worlds. Do you understand now?