The Sennheiser HD 450BT and HD 350BT headphones are both wireless over-ear headphones, but there are a few key differences between them: 1. Sound Quality: The Sennheiser HD 450BT headphones have larger 40mm drivers, compared to the 32mm drivers of the HD 350BT headphones, which may result in slightly better sound quality and a wider soundstage. 2. Battery Life: The Sennheiser HD 450BT headphones have a longer battery life than the HD 350BT headphones. The HD 450BT headphones offer up to 30 hours of battery life, compared to the 20-30 hours of the HD 350BT headphones. 3. Noise Cancellation: The Sennheiser HD 450BT headphones have more advanced active noise cancellation (ANC) capabilities than the HD 350BT headphones. The HD 450BT headphones have three ANC modes, while the HD 350BT headphones only have one passive noise isolation mode. 4. Price: The Sennheiser HD 450BT headphones are more expensive than the HD 350BT headphones, as they offer additional features and improvements in sound quality, battery life, and ANC. Overall, the Sennheiser HD 450BT headphones offer some improvements in sound quality, battery life, and ANC over the HD 350BT headphones, but at a higher price point.