That very much depends on your needs. Any sound that can be programmed on Spectrum can be loaded into the C4. However, the two pedals differ when it comes to the controls for changing that sound on the fly. By this I mean that, due to the extra available coding space in the Spectrum (itself a result of its having fewer voice and (volume) envelope options, etc. compared to C4), Source Audio were able to add some extra options into the list of parameters that are controllable by the Spectrum’s two control knobs and by its external controls; these options are not available in C4.
Relatedly, another consideration is how likely you are to want to manipulate or change the sound using the surface knobs; the Spectrum’s bottom two knobs control four parameters (two are hard-wired and two are user-assignable), whereas the C4’s bottom two knobs only control two (user-assignable) parameters.
Another thing to think about is how you program the two pedals. Because Spectrum is simpler in regards to having overall fewer options, I find that my approach to programming the two pedals can differ. I’m finding that the Spectrum pushes me towards exploring sounds that I might not otherwise have done had I been using the C4. With the C4, I’m focused on the oscillators and make use of the sequencers, FM capabilities and amp envelopes. On the Spectrum I’m finding I’m more likely to explore the distortion engines.
Certainly there are enough sounds available to fill both pedals with a completely different set of 128 sounds, so if you’re likely to use that many and/or you want the extra control destinations, it’s easily justifiable. Plus it’s purple and looks well sexy.
I’m very curious as to the numbers of each pedal out there (and who uses both). @rsmith601 if you are unable to cite actual numbers of each pedal sold for business-secrecy reasons or the like, would you be able at least to give an idea of the proportion of C4 to Spectrum units out there?