What's the difference between iShowU Instant and iShowU Studio?

The main difference is that iShowU Instant is a realtime screen recording app - meaning that the recording all done and ready to be used as soon as you finish recording. iShowU Studio is a complete screen recording and editing suite with a lot more post-recording options, but it takes time to create the final recording. For a feature-by-feature comparison have a look at the website.