As for a comparison, Optimizely uses Akamai which has 100,000 nodes in 75 countries. VWO uses Cloudfront and does two round-trips. Cloudfront has 32 points of presence in 12 countries.
Optimizely response time: Slowest: 289ms, Average: 65ms, Slowest: 19ms VWO response time: Slowest 1983ms, Average: 276ms, 171ms
Source: http://till.klampaeckel.de/blog/archives/100-Shopping-for-a-...
Unique features to Optimizely that VWO does not have:
- Mobile web support & editor
- Dynamic text insertion
- Staging server support
- Human-readable results
- Documented API
- Advanced developer tools
- Cross-browser testing for Chrome, Safari, Firefox, IE, iOS, and Android
- One-click integrations with Google Analytics, Omniture SiteCatalyst, KISSmetrics, MixPanel
- Implementation is 1-line of code vs 21 lines of code
- Pricing starts at $19/mon vs $49/mon
- Optimizely comes with a 100% money back satisfaction guarantee with 99.999% service level agreement
- Optimizely support offers web, phone, email, and a dedicated account manager, as well as the CEO's direct cell for Platinum customers
In terms of adoption by the top 10,000 websites on the internet Optimizely is 6x bigger than VWO.
Sources: http://trends.builtwith.com/analytics/Optimizely http://trends.builtwith.com/analytics/Visual-Website-Optimiz...
Note: I co-founded Optimizely and used Google Website Optimizer, Omniture Test&Target, and VWO but by no means an expert in their products.