Sahidic is one of the oldest dialects of Coptic language. There is a wealth of manuscripts written in Sahidic Coptic, including the Nag Hammadi manuscripts, Bible, Apocrypha, Gnostic text, the writings, of St Pachomios & St Shenouda Archmandrite, older liturgies were written in Sahidic. A few traces of Sahidic Coptic can be found in current prayers.
It is said by Prof Christian Askeland in one of his lectures on Coptic manuscripts.. That Sahidic was more or less the official dialect, it relies heeavily on consononants with supralinear strokes to denote vowels. In this it has some degree of resemblance to Demotic Scripts where consonants were the ones basically written. Its usage extended through upper Egypt, with local dialects as Fayyumic in Fayoum, Lycopolitan in Assiut & Sohag, Achmimmic & SubAchmmimic in Akhmim & neigbouring areas.
Bohairic was the dialect used in Delta. By the 10th Century the Coptic Orthodox Popes came from Natire Valley, Shihat ϣⲓϩⲏⲧ The manuscripts were written in Bohairic, which eventually became the official dialect of the Church. Monastries were active in translation, and translated a lot of text to Bohairic.
In 1857-1860 Arian G. Moftah, proposed a change of pronunciation of Bohairic Coptic to match Modern Greek letters' pronunciation. This was adopted by the Church. In the late 1960s & 1970's Dr Emile Maher Ishak studied phonetics and came to conclude that that the Old Bohairic pronunciation is authentic.
Sahidic is different from Bohairic in usage of vowels, consonants, & terminology, even grammar is different to some extent. If you're interested in manuscripts, I'd say go for Sahidic. If you're interested in praying in Church then, Bohairic is the way to go. I'd learn Old Bohairic for its authenticity & aesthetics.