Instapage, Kickoff Labs and Unbounce offer A/B split testing. You can A/B split test between two landing pages and compare two versions of a page to see which one gives optimum results. It also determines the best version that engages your visitors more.
All three services (Kickoff Labs, Instapage and Unbounce) provide A/B testing in their Premium plan. They all provide unlimited A/B testing variants to improve your conversion rate.
You can compare them here.