Grammarly is a checker that follows you into browsers, Office, Google Docs and phones; QuillBot is a toolkit you visit, built around a paraphraser. Grammarly's free plan covers most people's error checking, while QuillBot's free plan is capped at 125 words per paraphrase.
These tools overlap less than their feature lists suggest. Grammarly's strength is that it catches mistakes everywhere you type without being asked: the browser extension and desktop app work in email, chat, documents and forms, and its suggestions are accurate and explained. QuillBot is a destination you visit to rewrite text, and its paraphraser's preset modes (formal, simple, shorter, expanded) are quicker than writing a prompt. QuillBot has a grammar checker too, but Grammarly is the better tool for checking errors across every app.
On price, QuillBot is the cheaper habit. Premium works out to about $8.33 a month on yearly billing and lifts the 125-word paraphrase limit, unlocks more than ten modes and adds plagiarism checks. Grammarly Pro is $12 per member a month on yearly billing, and trackers disagree on the monthly price, showing $15 or $30. QuillBot's price also varies by region and promotion, so with either tool, check the figure at checkout.
The larger threat to both is the general chatbot. Full-sentence rewrites, tone changes and text generation are now things ChatGPT, Claude, Gemini and Microsoft Copilot do inside the same apps, often for free. That leaves Grammarly Pro's case resting on plagiarism checks, style guides and team features from the old Business plan, and it means anyone already paying for ChatGPT, Claude or Gemini can skip QuillBot.
Students should read the small print on QuillBot. Paraphrasing someone else's work without citation is still plagiarism, many schools treat 'humanising' AI text to dodge detectors as cheating, and AI detectors, QuillBot's included, give false positives. Grammarly's concern is direction rather than ethics: since the Superhuman rebrand, Enterprise is pitched mainly as a route into Superhuman Go. For most writers the free Grammarly plan is the one to install; QuillBot Premium suits a student with a heavy load of summaries and rewording.