Sill vs Otterly AI: GEO Monitoring and Experimentation Compared

Otterly AI ($29-$489/mo) tracks brand mentions, Share of Voice, and citations across ChatGPT, Google AI Overviews, Perplexity, and Copilot, with Gemini and AI Mode available as paid add-ons ($9-$149/mo depending on tier). Its GEO Audit evaluates pages against 25+ factors covering fluency, authority, and technical structure. The product serves agencies and in-house teams with a monitoring-first approach: track visibility, audit pages, identify gaps.

Sill (Free-$225/mo) monitors across ChatGPT, Perplexity, Gemini, Google AI Overviews, Claude, and Grok at every pricing tier with no add-on fees. Beyond monitoring, Sill generates GEO recommendations based on monitoring data, monitors brand facts via Watchdog (flagging AI hallucinations and inaccuracies), and offers an experimentation layer to measure citation shifts with statistical controls, per-platform models, and calibration for false positive rates.

Otterly's GEO Audit evaluates pages against 25+ factors including fluency, authority signals, and technical readiness for AI extraction. The product is well-regarded: Gartner Cool Vendor 2025, G2 rating of 4.9/5, and notable clients including Adidas (+7% SOV, +2.43 brand position improvement). The agency partner program with 50+ certified partners makes it a strong fit for agencies managing multiple clients with reporting needs. Looker Studio integration supports agency client deliverables.

Otterly's March 2026 research experiments produced some of the most valuable public GEO findings. The llms.txt experiment tested whether deploying an llms.txt file affected AI bot traffic; after 90 days, only 84 of 62,100+ AI bot visits targeted the file, performing 68% worse than average content pages. The "BE VISIBLE" experiment created a fictional GEO agency with a minimal 7-page website, no blog, no backlinks, and no domain history, then tracked whether off-page citations alone could generate AI visibility. The result: rank #7 in ChatGPT within 14 days, with 76% of mentions coming from location-specific, year-tagged prompts.

These were internal research projects, not features available to Otterly customers. The distinction matters: the product is monitoring and auditing; the experiments were one-off studies published on their blog. As we discussed in our GEO tactics analysis, these experiments produced findings that before-and-after monitoring cannot. Entry pricing is competitive at $29/mo for 15 prompts across 4 platforms.

Sill includes ChatGPT, Perplexity, Gemini, Google AI Overviews, Claude, and Grok at every pricing tier with no add-on fees. Where Otterly charges separately for Gemini and AI Mode access, Sill bundles all six platforms into the base price. This matters because 91.6% of cited URLs appear on only one platform; monitoring fewer platforms means missing visibility shifts that happen on the platforms you are not tracking.

GEO recommendations are generated based on monitoring data: 748 recommendations across 62 brands to date, with 87% being on-site fixes that teams can implement without third-party coordination. Where Otterly's GEO Audit evaluates what your page looks like, Sill's recommendations tell you what to change and why, grounded in the specific citation patterns observed across platforms.

Brand Watchdog monitors what AI platforms claim about your brand against a fact profile, flagging inaccuracies and hallucinations. With ChatGPT fully correct about brands only 59.7% of the time, automated fact monitoring is a defensive necessity.

The experimentation layer uses statistical controls to measure whether content changes shifted AI citations. Affected prompts are compared against unaffected prompts for the same brand. The system fits models independently per AI platform, surfacing when a change moves one engine but not another. Built-in calibration establishes empirical false positive rates. First experiment: 6-8 weeks; subsequent: 2-4 weeks. Experimentation requires 25+ prompts.

Choose Otterly if you are an agency that needs a mature partner program, a structured 25-factor audit for client deliverables, and a product that has earned Gartner recognition and strong G2 reviews. The $29/mo entry tier is the lowest paid starting price in AI visibility monitoring. Looker Studio integration and multi-client management tools make it the strongest option for agency workflows. The Adidas case study (+7% SOV, +2.43 brand position) provides a concrete reference point for client conversations.

Choose Sill if you are an in-house marketing team or brand that wants all six AI platforms included without add-on fees, actionable GEO recommendations rather than audit-only reports, and a path from monitoring to experimentation. At 100 prompts, Sill costs $90/mo with 120 prompts across 6 platforms; Otterly costs $189/mo for 100 prompts across 4 platforms. Brand Watchdog provides automated fact monitoring that Otterly does not offer. The experimentation layer measures citation shifts with statistical controls, giving teams evidence beyond before-and-after observation.

Sill's free tier includes monitoring across all 6 AI platforms, GEO recommendations, and Brand Watchdog. Experimentation requires 25+ prompts, available from the $90/mo Basic plan.

For the full landscape of GEO testing and experimentation tools, see our hub comparison of every GEO testing tool in 2026.