Convert Plus and Convert Pro are two different plugins developed for the same purpose of lead generation. Convert Plus is a simple and basic lead generation plugin that comes with a point and click editor with fixed layout templates that can be edited as per your needs. It is built with single site users who need just the necessary features for lead generation.
Convert Pro is further an advanced version that comes with a drag and drop editor giving you complete design freedom. Convert Pro has more features like multi-step optin forms, Google Analytics integration and more. This is built for marketers and users who wish to use some advanced lead generation techniques and track campaigns.
Apart from these, Convert Pro is optimized for speed and performance as well. Please go through this blog post to learn more about the difference between the two.