VWO is a great product but here are some of the unique benefits of using Optimizely:

First is ease-of-use and simplicity: we hope Optimizely is a product anyone can use, not just IT or engineers. Paras Chopra, the creator of VWO recently said on Quora: "Optimizely is a great tool and perhaps even bit easier than VWO, so I would recommend you to signup for a free trial of both the products and see which one is suited best to your needs."

Secondly, we enable power-users: all of the changes you make in Optimizely can be transformed into JavaScript statements that correspond to those changes. We enable you to see the JavaScript that gets generated and even edit it or write your own if you want to create sophisticated transformations of your page (click the "Edit Code" button in the bottom-right corner in the editor).

Third, we enable you to track any numbers of goals, even if you decide you want to measure a goal AFTER you've started your experiment. You can track any pageview, JavaScript event, or custom event and match these goals exactly, using a substring or regular expression. We keep all of the data you've sent us so you can always add and remove goals even if the experiment is still running. This gives you the flexibility and power to measure any goal you have on your website in real-time.

In addition to these three we are also unique in offering automated cross-browser testing, behavioral targeting, undo/redo, and unlimited simultaneous experiments.

I'm the founder of Visual Website Optimizer.

While Optimizely is strictly A/B testing, VWO is a much more comprehensive testing tool. In addition to A/B testing, we support Multivariate testing, Split URL testing, Heatmap/Clickmap, Targeting/Segmentation and Revenue Tracking. VWO is being used by companies that have sophisticated and comprehensive testing needs (like AMD, Microsoft, Groupon, etc. and thousands of SMBs)

That said Optimizely may be slightly easier to use but with VWO our focus is on both: comprehensiveness of feature set _AND_ ease of use. Since both companies offer free trials, best is to signup for both of them and see which one is better suited to your needs.

Without diving into the arcane specifics, I can safely say that I wouldn't have my sanity today without Optimizely.

It's mind-numbingly simple to use, but also hugely powerful, and intelligently implemented. Many people seem to assume the power/appeal is in the visual editor, but they've advanced so far ahead of just being "visually created experiments", in both experiment design, targetting, implementation, and customization; they've entirely eliminated the headaches and fudging that usually comes with online testing tools. I'm as happy of a customer as they come.

I spent the last month using VWO, and despite the website needing a slight Web 2.0 visual update, it has been a good experience.

I'm now getting started with Optimizely and the biggest thing I miss is Video demos. I also liked that VWO had a much smaller JS file with the option to remove Jquery. Fix this Optimizely, I don't need to load Jquery twice!