This is another question of formality and register. These phrases are the same in meaning, but "every so often" is more casual than "every once in a while". Some might even consider "every so often" to be fully colloquial (slang). Technically, the only fully formal version of this would be "once in a while", since "every once" may still be considered colloquial. The other difference is that, while "every so often" is a set phrase, "every once in a while" can be modified for intensity: "I see the president of our company only every once in a great while." "Every once in a long while, I treat myself to dinner and a movie alone."