Signal and Threema are both great choices as far as a secure messaging app is concerned. While ultimately it may come down to opinion, which is more secure: Signal or Threema?
Signal
Advantages: Well-known and thoroughly tested protocol; disappearing messages
(Potential) Disadvantages: Servers are on Amazon AWS; Questionable inclusion of MobileCoin crypto wallets
Threema
Advantages: Swiss-based with their own servers; No mobile number required
(Potential) Disadvantages: Latest "Ibex" protocol still in its infancy