Answer from halachafortoday Web site-
There are two different Minhagim. One (L’Omer)is a count down “from” when the Korban
Omer was brought while the other (B’Omer) is in reference to how many days “in”
Sefirah we are in.
Most Poskim go with L’Omer. However it is only L’Chatchila, as either one is acceptable,
and even if it isn’t said at all, rather you just said “Today is the 21st day which
is three weeks” is Yotzei. See Mishna Berura 489:8.
It would probably be best to say the same Nusach each night. The Nusach that your
father (or husband, if you’re a married woman) says is the one you should follow.