Both are solid choices, both have good security teams and release functioning well designed software. Is Proton & Signal immune from bugs/exploits? No, but they do a much better job in respecting and protecting your privacy than many others and fix bugs quickly. Unless you use PGP in Proton email is insecure and can be tracked and scooped up for surveillance, then you have to worry about meta data in emails. Signal doesn't have to worry about those issues of clear text or learning to use PGP, E2EE is baked in so it's more secure by design than email or Proton if used wisely.