I don't think the cabbie's play is just a 50/50 chance game - he is portrayed to be a sophisticated serial killer and no such killer would use pure chance to conduct the murder acts. Sure he can be great at reading people's minds but there is still a huge risk, especially when he has conducted four murders and survived. As the cabbie said himself: after four games, he survived. It's not chance. It's genius.

So here is my theory:

Both pills are indeed identical and both are poisonous. But the cabbie takes an antidote shortly before or after taking the pill. That way, his victim will surely die and he will surely live. The real correct choice is not to choose which pill to take, but to choose the gun, which Sherlock did.