I have the QC20's along with the QC15's. They're both great and for my ears they provide the same noise reduction. The 20's have a rechargable battery and a button that turns off a portion of the noise reduction to hear people speak without taking them off.
For travel by plane or car the 20's are much better just because they're smaller, lighter and fit in the ear. You don't have to deal with hearing sounds when headphones touch the seat. On the plane it removes almost all of the noise, especially the engines. I can't even imagine flying without them.
Contrary to belief they don't remove all sounds. They work really well with lower to mid frequencies. High frequencies not so much. It's like putting ear plugs in your ears but you have the option to play audio if you like.
They are a bit expensive but if you value quiet they're well worth the money. Make sure you get the ones for Apple devices. They include all of the buttons needed to work with iPhones, iPads...
I've not used them in the car but on a flight they remove or dull most of the noise so they should easily block out road noise.
Had them for a few weeks and returned them because of the battery unit. Just a very poor way to connect with a short pigtail.
Apart from the battery pack, was everything else ok though?
I have a set as well and the battery pack and the small pigtail are an annoyance. However, I remedied that by purchasing a extension cable from Bose.
That effectively moves the battery pack more towards the middle of the cable run. This makes things much easier to work with. I use them for plane rides, and they work great. I also have the QC15's and those are really good as well.
For the first question about blocking road noise they work really well for that.
In terms of which one is better at noise cancelling I would say the QC15/QC25 are slightly better because they cover the entire ear. This is just for no audio playing thru the headset. Also, if you wear glasses there's a slight bit of sound leaking around the frame.
The QC20 sounds better to my ears when music is playing. I also find them more comfortable to wear for long periods of time because of the weight and the lack of pressure.
For the battery pack I use a small hair tie to attach it to the back of the phone and/or an audio extension cord depending on what I'm doing at the time.
This review pretty much sums up what I notice.
http://thewirecutter.com/reviews/best-noise-cancelling-in-ear-headphones-so-far/
Which are better at cancelling out background noise, the QC20s or the QC15s?
Apart from the battery pack, was everything else ok though?
QC 15's by far. While the QC 20's do well, the over the ear capability of the QC 15's along with the noise cancelling features are much quieter overall.
Oh... now you've thrown a spanner in the works. So it's the QC25s or the QC20s.... dammit. Having said that I own many over the ear headphones which surpass the bose sound quality wise so the QC20s are still my preference I think.