SMS is transmitted plain text, can be snooped on by hackers, saved by the telecom companies or intercepted and saved by governments. Although I think sometimes they just collect metadata, not the texts themselves, but I don't know. Same goes with voice calls. Signal is E2E encrypted, so all those worries are negated as long as the encryption isn't back doored. So id prefer to use signal. I'm not aiming for complete anonymity, just peace of mind that I can reasonably assume my digital communications with family and friends are with just them. And no digital record of those communications will be saved on someone's servers for eternity.