If your two main criteria are security support and updated kernels, I recommend installing Debian 9 (stable) and using the backported kernels — kernels which enter testing are regularly backported to stable. If you run Debian testing you need to keep an eye on security updates yourself; barring exceptional circumstances they are only pushed to unstable, and migrate to testing following the testing rules (see the Debian wiki for details), which means that in some cases they can take a while to become available. Paradoxically, as the Debian 10 release gets closer and the freeze comes into effect, this will become more of a problem (but the freeze is one of the circumstances in which security fixes are more likely to be pushed to testing explicitly).
To install such a kernel, add backports to the available repositories:
echo deb http://deb.debian.org/debian stretch-backports main | sudo tee /etc/apt/sources.list.d/stretch-backports.list
then update your package indexes:
sudo apt update
and install the latest backported kernel (here, for
sudo apt install -t stretch-backports linux-image-amd64