Depends on how you'd use them. If you'd use them outdoors then it's the P7. In a quiet room with a good amp and other equipment the HD650 is better for dedicated reference listening.
I'd go with the HD650 for reference listening at home and just use an IEM outdoors. Easier to carry around, blocks more noise, depending on which portable headphones you have to consider the IEMs might be easier to have the eartips swapped out than the earpads (much less the batteries on ANC and wireless headphones), etc.