I couldn't find any direct comparison, it looks like the IETF standardization work didn't even consider it worthwhile comparing to iSAC as it was judged outdated and "being phased out". (For example, Google participated in the tests, but they only tested against iLBC, not iSAC)

So it's likely that Opus is way better. It can certainly scale to much higher quality than iSAC can just by the format alone.

Qualitywise, Opus outperforms the state-of-the-art high-latency codec at music encoding while being a low latency music & speech codec itself. The only case where another codec outperforms Opus anywhere seems to be AMR, when encoding speech at very low bitrates (<= 6-12kbps). But AMR also has higher delay.

You can read the summary here: http://tools.ietf.org/html/draft-ietf-codec-results-01