I had a flexible sigmoidoscopy without sedation and it was the most uncomfortable feeling in the world, I hated it.
I then got a colonoscopy with complete sedation and it was marvelous. I had minor cramping afterwards but that was it.
I recognize there are risks with sedation, but they are minor, and there's also risks with having a tube up your butt. I also acknowledge that a lot of it is probably mental, but I personally would never do it without sedation UNLESS I experienced something like pulmonary arrest or creepy memories, etc. that made my mental priorities shift.
I would suspect the extra pain/discomfort from having a full conoloscopy vs sigmoidoscopy to be minimal to none. The most uncomfortable part is actually navigating the sigmoid colon, which is two sharp successive turns.