First of all welcome to the forum.
Coptic has seven major dialects only two of them are very widespread.
Sahidic is the richest of all dialects in terms of number of manuscripts and amount of vocabulary
Bohairic was the dialect adopted by the Church by the 11th century.
Almost all manuscripts written in Boahiric are from the 9th century and onward
In my opinion Bohairic is a bit easier to learn.
If you are going to study the Bible, its available in both dialects.
If you are going to study Coptic church payers Bohairic is better.
If you are going to study old teachings of Coptic fathers/saints Sahidic is better.
If you are going to study none christian text, Sahidic is better.
Hope that helps