Is There a Good Tool, Service or Plugin for Testing Opt-In Methods

I am looking to split test different forms for email opt-ins. Is there a consensus on why type works the best or is there a tool, service or wordpress plugin I can use for this? I'm thinking of testing a top-bar, pop-in, slide-in and content blocking opt-in.

Thanks for any direction on this,

Cafro

Thrive Leads is good, and so is Optin Monster.