My answer would be valium (diazepam) hands down!
You have every justification for requesting this before a colonoscopy.
It is much more effective, and safer in my opinion than a beta blocker.
Diazepam calms the mind and the body, making you feel genuinely relaxed.
Beta blockers mask the physical symptoms of anxiety such as sweating, trembling or a pounding heart but will not alleviate the psychological distress. They also lower heart rate and blood pressure which can be undesirable where these effects are not wanted.