Had a look at the Which? reviews for you.
QC15 - Best Buy 76% - Pros: Good sound quality across all genres, excellent noise cancelling works really well, no sound leakage. Cons: Noise cancelling relies on battery supply. Should I buy them? Yes, if you're looking for a set of comfortable, noise-cancelling headphones these are up there with the best.
Momentum - 53% - Pros: Look great, durable. Cons: Expensive, uncomfortable. Should I buy them? No. Sound quality might be above average, but if you're intending on wearing these for long periods of time there are more comfortable alternatives available.
Others you might be interested in as follows.
B&W P5 - Best Buy 71% - Pros: Very good sound quality, unique look, minimal acoustic leakage. Cons: Expensive, no noise-cancelling feature. Should I buy them? There are higher-scoring Best Buys, but the combination of great sound and unique design should be enough to put these on everyone's shortlist.
Bose AE2 - Best Buy 70% - Pros: Good, detailed sound, comfortable, durable. Cons: Some acoustic leakage. Should I buy them? These headphones are a worthy Best Buy and should be on your shortlist if you're in the market for an over-ear set.
Sennheiser PX 100-II - Best Buy 77% - Pros: Excellent sound quality, very portable, fold into a small package. Cons: Poor levels of sound leakage. Should I buy them? Yes. You'll do well to find a better set of headphones in this price range. If you have a bigger budget take a look at some of our other Best Buys, all of which perform better when it comes to sound leakage.