Given the choice, I would choose sex education over abstinence. Teenagers and pre-teens are engaging in sex already. There are some kids in grade school that are having or attempting to have sex. I think abstinence was a good approach when my generation was in elementary school (I'm 19, so back in the 90s and early, early 2000s). I can honestly say many of my classmates did not know very much about sex. Now it is everywhere.
Sex education is beneficial because it usually incorporates abstinence and prevention in addition to explaining the risks that are involved in having sex. If a child or teen is not told what sex is, they will probably explore on their own out of curiosity and find themselves in a horrible situation. If sex education is taught, they will know how to be curious safely. In addition, it costs the government less money! There is no point in spending money on abstinence-only courses when teens are obviously having sex. Shows like "16 & Pregnant" and movies like "The Pregnancy Pact" are based on society, not a writer's imagination.
I think sex education does have its downsides. It has the potential to take away from parents having their own version of the sex talk with their child. Some kids learn better from their parents. Another con involved in sex education is that if a teen was not curious before, they will almost definitely be curious after taking a sex class. Nonetheless, exposure to sex is inevitable. It should be taught instead of hidden.