Both are a good option. 

Signal has good encryption and well established to have plenty of people that you already know. The downside is the need for a phone number to sign up. There are ways around this but it being required is a red flag. A good choice if you just want to message privately but don’t care if someone knows who you are. 

Simplex is a fantastic new service. Excellent privacy and anonymity but still very new without a lot of users yet. Still has some bugs and isn’t the easiest for non tech savvy users. A good choice if you need privacy and anonymity.