Yeah you definitely want something with decent isolation versus something with noise cancellation.
I've owned a few sets of noise canceling headphones and really the only situation where they work, and work well is in an airplane when you want to sleep, in a server room where there's constant fan noise or when next to an industrial air conditioner. In these cases, its a god send, and will help prevent tinnitus and such.
Noise cancelling almost doesn't work in busses and trains. In a bus, a set of NC headphones might irritate you more then anything as a bus tends to make a lot more clicking and poping noises. Sure you won't hear the engine as much, but you will hear the tires scratching on the pavement, every single person talking with an elevated clarity and possibly something you've never really noticed like the warble of the bus windows flexing as the bus drives and rattles along its path. That last one was a new level of annoyance I wasn't prepared for
The good thing about noise cancellation though is that you can easily make use of it over a set of decent IEMs! Just put the IEMs in your ears, and put your choice of NC headphones over to remove the constant rumble that usually will penetrate your IEMs at least to some degree. So you can buy a cheap set of under 50$ NC headphones if you just want the kind of silence that only NC can offer... Some people swear by Bose but this trick is jsut as good and you get to choose the kind of sound you want rather then the terrible sound Bose thinks sounds awesome -_-