Try both, one at a time... and then both daily... decide... and then stick with the one *your* body prefers.
I have a friend who prefers sauerkraut... I tend to lean towards kefir.
BTW - to allay any fears... if you use a high quality raw milk,and a trusted source for kefir grains, you should never have any issue with it.
I have drank raw goat milk kefir made with two week old, raw milk, sat at room temperature for over three days (I forgot about that batch), and then stored for over two months with no ill effects.
People knew what they were doing before refrigeration... they had to in order to survive.
YMMV - I am pretty tough when it comes to stuff like that after years of this.