The first real question is; what kind of phone do you use? If you are an iPhone user then HomeKit and Siri is the obvious choice. As an android user things become a bit more complicated. Do you have Amazon Prime? Already own an Alexa enabled device? Then possibly staying under the umbrella of Alexa powered devices is the best move. As a Galaxy user or Samsung aficionado, Smart Things is yer another Alternative.
With HomeKit, you are sure that your smart home devices are secure. Apple does not take its “Works with HomeKit” certification lightly, often demanding that devices seeking accreditation would follow specific hardware and software with a chip that would handle encryption being part of the requirements.
This is the reason why there are only a few devices that are certified to work with HomeKit, but it also ensures that security is the least of your worries. Every communication between your HomeKit devices is encrypted, and Apple itself would not know what you ask Siri to do.
SmartThings, on the other hand, does not have this oversight. In fact, one of the things that draw most people to SmartThings is the homebrew integrations that the developer community creates for devices that are not official partners yet. If you are still not sure, call an expert. We are here to help.