The verdict
If you run an online shop, especially on Shopify, and want to start a testing habit without a big commitment, I'd pick Omniconvert.
If testing is a team's job, several groups run tests at once and engineers need server-side tests and feature flags, I'd pick Optimizely.
For a small B2B team, I would pick neither first. Omniconvert assumes a shop, and Optimizely's sales process and cost are out of proportion to a small site. Look at Convert Experiences or VWO instead. And remember that any test needs enough visitors to reach a result.