"They wanted to go to New England this summer; therefore, they've saved for a whole year."
"Therefore" is the conjunctive adverb; it connects the two independent clauses.
Conjunctive adverbs are used to show sequence, contrast, cause and effect, and other relationships.
Notice the punctuation: there is a semi-colon before it and comma after it.
Other examples of such adverbs are... otherwise, nevertheless, however, meanwhile, etc...
"So" (in the other sentence option) is a coordinating conjunction that (also) serves to connect the two independent clauses. Notice that only a comma is used after it.
Other examples are ... and, but, yet, or, for, nor.
