Q. Is it better to count sefira by oneself soon after the zeman begins to do zerizin makdimin and temimos, or wait until one davens maariv with a minyan later on?
A. Beer Heitev (489: 2), Chok Yaakov (ibid, 16), Chayei Adam (: 17), quotes from Sh’loh (P’sochim 3b) that on the onset, like other mitzvos, one should count sefirah “berov am” with the many, together with the tzibur. Additionaly, this echoes the harvesting of the omer, that was also done very publically and with the attendance of many.
However, Mishna Berura (489: 2) mentions to count early in the beginning of the (first) night, in order to comply with temimos or counting complete weeks requirement. Delaying for later would be a ‘less full’ counting and it applies also to the rest of the nights (Siddur Yaavetz, Shulchan Aruch Horav ibid. 3), Others also quote that the principle of “zerizin makdimin” or complying with a mitzva the earliest possible. (Mor Uketzia ibid.)
Nevertheless, Poskim (Minchas Yitzchok (9: 56; 2), Igrois Moshe (4: 99), Shevet Halevy 6: 53 and others), write that if one usually davens at a later ma’ariv minyan, one should count sefira at that time together with the tzibur. Three reasons are quoted. As already mentioned, the benefit of complying together with the many. The fact that m’aariv and k’rias shema are more common and usual mitzvos (todir) and therefore they preempt sefira. And the reality that since sefira is not that common, one may come to forget and neglect it.
Horav Shlomo Miller’s Shlit’a opinion is similar.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a