The strengths here barely overlap. One is better at finding the current answer and living inside your documents; the other is better at acting on an answer and covering a wider range of tasks. Choosing one means permanently giving up whichever of those you use second most.
In Chat Smith you can keep both. GPT and Gemini models sit in the same app, so you can send one prompt to both, compare the answers, and keep whichever holds up. Context carries over when you switch models mid-conversation, on Android, iOS, or in the browser, with Claude, Grok, and DeepSeek models there too.
On cost: ChatGPT Plus and Google AI Pro together run about $40 a month. Chat Smith Pro is $9.99 a month, or $39.99 for a year, and covers both families.
One honest caveat: a multi-model app gives you the models, not every feature of each subscription. Deep Research, Workspace integration, and the agent surfaces stay in their native products. If your work lives inside those, keep the subscription. If what you want is access to strong models without picking a side, the maths is hard to argue with.