Don't have all too much experience with synthflow but from a top level (having played with bland.ai, synthflow, and air.ai) vapi seems more developer friendly. Namely, they handle the framework and then let you process however you want in the backend, and I appreciated that. The only thing I think that was missing for me in Vapi was in-built knowledge base but we have that now too so...Vapi pretty much has everything I need