A:
You'll buy based on the software you need to run, to RAM.
RT will allow you to install only Modern/Metro UI apps which have lower RAM requirements, where 2GB is sufficient. Surface Pro will allow you to install Modern/Metro UI and desktop apps, where you'll usually want 4GB.
Example: my wife would love to use an RT for work because of the battery life. It has almost all of the apps she runs (Office, One Note, Skype, browser, Outlook, Kindle, etc.). However, there is one 3rd party app she needs for her job that runs only in desktop mode (Pamela Recorder) so she can't use the RT for work since she can't install and run it on RT. She would need the Pro.
We do have an RT for family use and love it.