Passive isolation will likely work for you if the outside noise will be more talking than constant frequencies. The two closed cans I used last semester were first the Beyerdynamic DT770, and then the Ultrasone DJ1 Pro (same as Pro 550). Each one does a pretty decent job at blocking out noise.
Here is a graph of the noise isolation levels of the two above headphones as well as the Bose QC15
The Ultrasone isn't quite as good as either, but with music I never had a problem with outside noise bothering me.
The DT770 is as good and even slightly better at isolation from 1K hz upward.
The biggest difference between passive and active noise cancellation will be in the lower frequencies, as is seen on the above graph.
Here is another graph with the Audio-Technica ANC7B instead of the Bose.
If you want the best noise cancellation, the Bose QC15 is hands down the best. If you can deal with only good-very good passive noise cancellation, there are several options such as the DT770, but the recommendation will largely depend upon what sound signature you want and what type of music you listen to.