STOP punctuation joins two complete sentences. GO punctuation joins a complete thought to an incomplete one. STOP is a period, a semicolon, a colon, or a comma plus FANBOYS. GO is a comma or no mark at all. The punctuation set is in ACT punctuation rules.
Two full sentences
If both sides could stand alone, you need STOP. A comma alone is a splice. FANBOYS with a comma is STOP, covered in FANBOYS. A period and a semicolon do the same joining job, which is why they cannot both be right in the same blank. That trap is in period and semicolon as the same job.
One side is not a sentence
Use GO: a comma after an opener, or nothing between a subject and its verb. Comma jobs are in ACT comma rules.
The bottom line
Read both sides. Two sentences: STOP. One sentence plus a fragment: GO.
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