Both Spiral and Duality use homomorphic encryption (HE). DPella, however, uses differential privacy (DP), which allows for a different set of applications.

As a rule of thumb, HE allows you to offload a intensive computation (or one that requires a private model) to a server that you don't trust. Only you can decrypt the results. DP, on the other hand, lets the server analyze big data and use the results. Privacy is achieved by each input being "noisy", so no little information is leaked. The statistics still work because over a large data set the noises cancel out.