Here's some more details for those who are curious about it.
잠시만요 and 잠깐만요 essentially mean the same thing.
잠(暫) means a short while, and 시(時) time, so 잠시 is "a short period of time".
잠 in 잠깐 is the same character, and 깐 comes from 간(間) which also means time(시간(時間) = time).
So etymologically, they are pretty much identical.
Historically, up until about two decades ago, 잠시 was used more in formal contexts while 잠깐 was prevalent in daily life. For example, 잠시 후에 (shortly) is one of the most common words on TV and radio.
But then something happened in the past few decades and 잠시 has gained an enormous popularity.
What used to be 잠깐만요 has become 잠시만요, and 잠깐만 기다려요 is now more often 잠시만 기다려요.
My guess is it has something to do with the modern trend of emphasizing service everywhere. The service sector probably noted 잠시 sounds smoother and more elegant than 잠깐 (깐 is a slightly jarring "tensed" sound) and started favoring it, and from there it spread to the general public as well. And while gaining popularity, it also expanded its scope, like saying 잠시만요 when passing through a crowd in place of the traditional 실례합니다 or 미안합니다. So people feel 잠깐만요 is not right in such newly adopted usages.
In summary, I would say you could use either in most circumstances. If you're in doubt, use 잠시, since it seems to be the new favored choice of many people.