A Colonoscopy examines your entire colon---about 5 feet long. A Sigmoidoscopy examine about 1/3 of your colon from your anus up the left side ----it's called the sigmoid section of the colon. The cleaning out procedure for both examinations is the same and is the worse part of the exams but most problems happen in the sigmoid/rectum section of the colon.
The other parts of the colon are the transverse(from side to side) and the ascending(from your right side where your appendix is up your rightside to the transverse section).
A flex sigmoid only examines the 1st 1/3rd of your colon and is often done without sedation. A colon however examines the entire colon to the terminal ileum and your are given sedation for this procedure. I would suggest doing the the colon, because cancer can occur anywhere in the colon, you are being cleaned out totally and you will at least be kept comfortable with the colonoscopy.