According to the OED, both consistence and consistency are valid nouns. Consistence originally had many meanings, which include:
- Standing or remaining still, quiescence; state of rest. Obs.
- Continuance, endurance; continuing state
- A settled condition of affairs. Obs.
- concr. Matter dense enough to cohere. Obs. or poet.
- The degree of firmness with which the particles of a substance cohere; degree of density. (Usually of more or less viscous liquids.)
As you can see, many of its definitions (of which there are 8 listed) are now obsolete. The last recorded use of consistence to mean consistency was in 1878. Consistency has now taken over many of these meanings, and is used far more often than consistence. Using consistence would not be incorrect, but it is an archaic form of a word that many people would not recognize today.