Both are interchangeable in most cases, but... 1. "잠깐만요" sounds a bit stronger because of the double consonant 'ㄲ' in '깐". 2. When you say "잠시만요", it indicates you need more time as Nam said above, on the other hand, "잠깐만요" implies "just give me a minute, please" literally. 3. "잠깐만요" is also more often used to get someone's attention in an informal and casual way. \^o^/

잠시만요 = 잠깐만요 ^^ It's the same meaning.

잠시(暫時), 짧은 시간 잠깐, 잠간(暫間)이 된소리화 하여 굳어진 말. 매우 짧은 시간. 잠시만 for a while, 잠깐만 for a moment., 거의 같은 의미로 쓰이지만, '잠깐'이 '잠시'보다 더 짧은 시간을 의미합니다.