While both are good solid effects packages, there are many filters in the Boris Continuum Complete (BCC) package (our only offering for the combustion host environment) which are unavailable in the Sapphire package such as Motion Key foreground object removal, or Optical Flow based image retiming to name just a couple. There are features built-into most filters in the BCC package which are not found in any of the filters in the Sapphire package, such as motion tracking and the PixelChooser masking and matting system. Also, all BCC filters include a pop-up preset manager that comes with about 1500 presets. The preset manager also enables you to save your own custom effects settings that can be reused on different projects or shared with other systems that also have BCC installed (FCP, Flame, Smoke, Avid etc).
At the end of the day it comes down to what you really are looking for in terms of an effects package. I can't tell you which one to buy (my opinion is obviously biased seeing as I work for BorisFX) but can point to some of the differences between each of these packages, which is what I have done above. You might want to download the trial version of the BCC product and use it for a couple of weeks to see if it meets your needs.