Security rarely comes in absolutes. Whatsapp doesn’t appear to be open source. That alone makes it security wise a worse choice than Signal.
That’s another viewing angle coming to the same result :)
But even before Fcbook purchased Whatsapp, it’d have been the worse choice than Signal.
Nowadays Whatsapp is a very bad choice in a lot of ways except for network effect, which is the only real strength of it.
Speaking of network effect: back when Signal was TextSecure, I could message a total of 2 (two!) contacts with it and the UX was far from being awesome.
Signal has come a long way. The UX is great and I can message a lot of my contacts on Signal now.