I would not declare Gemini or ChatGPT the marketing winner in the abstract. I would run both on my real tasks: ad variants, landing page drafts, email subject lines, briefs, and research summaries.
Where differences show up
Context length, tone, integration with Google or Microsoft tools, image handling, and how much editing you need for your brand voice.
Those change by task.
ChatGPT habits
Many marketers already have prompt libraries and custom GPTs.
Switching costs time.
Gemini habits
Useful if you live in Google Workspace and want research tied to that ecosystem.
Still needs editing.
My practical rule
Pick one primary for the team.
Keep the other for second opinions on high-stakes copy.
Log failure modes: hallucinated stats, bland CTAs, off-brand tone.
The best model for marketing is the one your team edits least while staying accurate. Fan debates are noise.