When two main clauses are joined together without the use of a coordinating conjunction, should the clauses be separated by a semicolon, a comma, a colon or a subordinate clause?
Don't join them.
It is easiest to make the two clauses separate sentences.
The two main clauses can be joined by a coordinating conjunction. By doing this, you make it easier for the reader to follow your thought.
It is possible to join two main clauses by a semicolon, but the clauses must have the same importance and should be of about the same length. Most people misuse the semicolon, so if you aren't sure whether the thoughts in the two clauses are of equal weight, it's best not to use it.