Hello Dave. When choosing between kvass and kefir for your okroshka, it really comes down to the flavor profile you prefer for your backyard meal.
Kvass offers a traditional, tangy, and slightly sweet bread flavor that is very refreshing on a hot day. Make sure to use traditional un-sweetened bread kvass, not sweet commercial soda.
Kefir provides a creamy, tart, and smooth base. Many people prefer to dilute kefir with a little sparkling mineral water to get the right consistency.
Both options turn out great, so choose kvass for a classic taste or kefir for a creamier soup.