I prefer kefir to yogurt because yogurt only has a 2-5 strains of beneficial bacteria, none of which aggressively attack and destroy pathogens in the gut.
Therefore, in a gut that is dominated by pathogens and candida (as in someone who has taken many rounds of antibiotics and other prescription drugs over the years and/or eats primarily processed foods), yogurt tends to have only a temporary impact in rebalancing the bacterial colonies in favor of the beneficial strains. On the other hand, homemade kefir has about 30 beneficial strains of bacteria and yeasts which do aggressively recolonize the gut by destroying pathogens. As a result, kefir has the potential to permanently alter the gut environment for the better.
You can sometimes get live kefir grains from a friend, as the kefir grains grow slowly over time and extras can be given away (or even eaten as a live probiotic!). If you cannot find any live grains in your community, you can mail order them from Cultures For Health.
How to Make Kefir