Can I use either environment now since I've downloaded the RobotC firmware, or do I not to reload the Arduino boot loader? I would like to program with both environments but for different purposes. I'm not trying to create one program with both. I just want to be able to use either at different times.

What do I have to do if I have to do something to restore using Sketch?

Edit 1: I found my own answer. I just went to Sketch and downloaded a program to my robot with no trouble. Returning to RobotC I had to download the firmware again before it would take a RobotC program. Not too difficult to do IMO.

What advantages did you find that the RobotC has over the regular sketch environment for Arduino? I am considering purchasing it for my classes.

Robot C has very good debugging/diagnostics tools to watch your code, motors, and sensors in action. You download your code and set break points, watch variable values, see sensor and motor values, etc.... That is the greatest strength of Robot C. I can prototype much quicker with Robot C. With Arduino I have to write individual Serial.print strings to the monitor in order to debug. That is the one big drawback with Arduino sketching.

Another the downside Robot C has is that you cannot utilize all digital and analog pins as you can through coding with the sketch compiler. I need to be able to create pwm for any need (e.g., piezo) and Robot C for Arduino is too limited for me at this time.