Obama asserted something quite different in last night's address. He stated that the Syrian army distributed gas masks to their soldiers before the attacks, that documents show Syrian generals reviewing the results after the attack, and that they planned to continue and expand the attack after the initial wave of gas was used. I would tend to believe Obama because he is much more credible than Putin, and has much more to lose if he can't produce evidence to support his claims.
Despite Putin's call for transparency, the fact remains that he is unwilling to tell the truth about what happened. Syria broke one of the most important international laws that we have, which is the ban of chemical weapons. If they aren't punished, then we are telling the world that it is OK to use chemical weapons.
Putin claims that the United States' position with Iran will be weakened if we intervene. The opposite is clearly true; our prior negotiations with Syria over their acquisition of chemical weapons in many ways mirrors our ongoing negotiations with Iran. We told Syria there was a red line and they crossed it. If we don't intervene in Syria, we are effectively telling Iran that our word is meaningless.
Putin's claims that an intervention will cause an escalation of the conflict and more terrorism are simply not true. The goal would not be to upset the balance of power or to cripple the Syrian army in any way.
Our objective would be to prevent the Syrian army from using chemical weapons again, and show the world that chemical weapons are unacceptable. Intervention would lower the risk of chemical weapons being used in the future, help discourage nuclear proliferation, and prevent the U.S.'s credibility in the region from being destroyed.