Caching is only useful for speeding up the loading of the same file, over and over, like your OS, so neither of your listed solutions will really speed rendering up.
For fast rendering, it is important to have AT LEAST 2 drives: one to read from and another to write to. If you try to do both tasks simultaneously on only one drive, it leads to the problems that you are seeing. Some even recommend 3 drives.
If you are otherwise satisfied with the WD Blue, then get another to separate your read/write activities. I can highly recommend the Seagate ST1000DM003 1 TB HDD that I have in my system and I believe that, according to manufacturer's specs at least, it is slightly faster than the WD Blue.