Both are the same in meaning, but "잠시만요" is a bit more formal than "잠깐만요". (You can ignore this difference.:) It's because "잠시" is sino-Korean version of "잠깐". Sino-Korean words are usually used in formal speech in Korean. \^o^/

의미 차이가 없습니다. 다만 '잠시'는 한자어이고 '잠깐'은 아니라는 것 정도일까요? There is no difference in meaning. However, '잠시' is the Chinese character and the '잠깐' is not .