Maybe this will help:
Sigmoidoscopy Screening Misses Cancer More Often - National Cancer Institute
"People aged 50 and older commonly develop small growths on the inner wall of their colon or rectum that may or may not turn into cancer."
"Colonoscopy is the most effective screening tool but it is also the most invasive and expensive"
"Less effective but also cheaper and safer is flexible sigmoidoscopy, which also inserts a lighted tube into the colon but less than half as far, leaving the upper (proximal) part of the colon unexamined. Both methods risk puncturing the colon, but the risk is somewhat greater for colonoscopy."
Results of the study:
"Advanced neoplasia were detected in 72 (4.9 percent) of the 1,463 women. Of these, researchers determined that only 25 (1.7 percent, an absolute difference of 3.2 percent) would have been diagnosed as having advanced neoplasia if they had been screened with sigmoidoscopy alone.
In other words,
47 of the 72 women (64.8 percent, or two-thirds) who actually had advanced colorectal neoplasia would have been told there was nothing to worry about if examined by sigmoidoscopy alone. The comparable percentage of men who would have been falsely diagnosed from VA Cooperative Study 380 was 33.7 percent – half that of the women. Researchers didn’t know what accounted for the gender difference."
So, there you have it, perforation issues exist with both, but are lower with a sigmoid. I'm sure there's also some risk with sedation involved with the colonoscopy. However, the risk of perforation according to these people:
eMedicine - Colonoscopy : Article by Jennifer Lynn Bonheur, MD
"The risk of perforation of the colon is 0.2-0.4% after diagnostic colonoscopy and 0.3-1.0% with polypectomy. A higher rate (4.6%) is associated with hydrostatic balloon dilatation of colonic strictures. Perforation is more common (1) in patients who are oversedated or under general anesthesia, (2) in the presence of poor bowel preparation, or (3) with acute bleeding, and generally results from mechanical or pneumatic pressure or from biopsy techniques."
So, compare a 0.2%-1% risk of perforation against the fact that, in men in an at-risk group, the sigmoid caught the cancer 33% less. That's quite a gamble, especially when it's your life.
