differences between the southern and new england colonies by the middle of the 18th century?

I can tell by your other questions you understand the answer to your own question. Therefore, I'll provide a very short answer. I'm not sure what your looking for other than provoking thought, maybe?

North-

Industrialized

Great infrastructure

Urbanized (i.e. New York, Philadelphia)

Specialized/ Diverse

South

Agricultural based

Dependent on Cheap Labor

Sparsely Populated (Whites)

Rural

Lacked Diversity (Dependent on Cash Crops)

There are others. Essentially, above is the list that would pave the way for Civil War. The War was mostly an economic and ideological decision.

They had teeth in the north.