- Pro allows you to scale your applications to be able to support more than one user at a time (as Laz said), which is a really important feature if you plan on having your apps used by multiple people.
- It also lets you easily monitor your applications usage with a nice user friendly dashboard that shows you how your resources are being used and how many people are using which apps.
- Another very useful feature with Pro is that it gives you the ability to authenticate users - meaning you can place any shiny app you have behind a mandatory "log in" screen so that only people in your organization will be able to access the apps.
- And lastly, as Laz said, you will get priority support from RStudi and I can tell you first-hand that their support is absolutely fantastic and they'll go beyond your expectations in trying to help you if you have problems
I don't think you can easily have both versions running on the same machine, usually if you install Pro it will overwrite the free version