Although I continue to believe that the U.S. should not intervene in Syria's civil war given the lack of critical U.S. interests involved, I believe odds probably favor the Syrian Army's use of chemical weapons. There is a degree of uncertainty, so I could be wrong. My thinking is as follows:
President Assad's defense against the claims of his Army's using chemical weapons is that it had no need to use such weapons in a conflict that appeared to be slowly turning its way. With a UN team in place to investigate previous reports of small-scale use of chemical weapons, resorting to such weapons would have been particularly risky. Yet, scenarios for such use do exist. Under battlefield pressures, emotions can override reasoned judgment. One possibility might be that a Syrian army unit, with tactical authority to use such weapons, attempted to achieve a battlefield breakthrough. With the rebels having engaged in de facto “human shielding” by placing military objectives in close proximity to civilians in the past (http://www.ohchr.org/Documents/HRBod...C.22.59_en.pdf), it is possible that the Syrian army unit targeted rebel forces rather than engaged in indiscriminate bombardment or deliberate targeting of civilians. All three possibilities--human shielding, indiscriminate bombardment, or deliberate targeting--would result in high civilian casualties.
An alternative scenario would involve the anti-Assad forces deploying such weapons. There have been past reports of their possible access to and use of sarin (BBC News - UN's Del Ponte says evidence Syria rebels 'used sarin'). The major argument against such a scenario is that they lack the sophistication and means to deliver the kind of fairly large-scale attack that took place. A counterargument would be that they have been receiving more sophisticated missiles and other weapons in recent months so they might well have gained a capacity to deliver a larger-scale attack
My best guess is that Syrian army forces very likely used such weapons in the attack in question, plausibly with an army unit responsible for the tactical decision rather than President Assad. With rebel units having a history of operating in close proximity to civilians (a war crime in itself), that situation is probably as plausible an explanation for the high toll suffered by civilians as indiscriminate fire or deliberate targeting.
Had the rebels resorted to the use of such weapons, they would have been fired at the Syrian forces. While some fire might have missed, it is inconceivable that every missile fired by the rebels would have missed its target and impacted the affected area. Right now, I've seen no reports that a wider area was impacted in the attack in question. Despite the anti-Assad forces’ lack of regard for civilian protections—something they share with the Assad regime—it is probably very unlikely that the rebels would have deliberately attacked residents who back them. Either of those scenarios would be necessary to give the strongest support to the notion that the rebels were responsible for the attack. Additional scenarios exist, but the cumulative probability of those scenarios is probably very low and probability of individual scenarios even lower.