I don't see the point in this argument, but I'll chime in anyways:
I think the QC15 are unbeatable when it comes to ANC, and nearly unbeatable when it comes to comfort (there's a sole Sony model I would hold up against them, when they were in production they cost about the same, but are completely open-back, so they provide no isolation). The sound from them is fairly good as far as a sealed headphone goes - there's no harshness or sibilance up top, the midrange is fairly clear, and the bass is not overly boomy or muddy. They're relatively quick and offer a decent sound-stage. They also extend very well (both up and down). That said, they do have somewhat prominent bass, and I suspect it may border on excessive for many users; especially if your frame of reference is one of the Sennheiser 5x5 cans. Now regarding the Sennheisers - I've heard the 555 and 595, and all that comes to mind is "boring" in terms of their presentation. Don't get me wrong, they're good, but they're very...bland.
I think for what your father is using his headphones for, they are the absolute best possible choice (apart from custom-molded high-isolation IEMs, which can exceed $1000). I also think that for at-home listening, the open-back Sennheisers are probably a better choice simply because they cost less, and I feel that paying extra for the ANC if you don't need it is wasteful (you also have to factor in that the QC15 eat batteries). In other words, you've both found a good match for what you need the headphones to do - but comparing them directly is like comparing a snow plow to an ambulance, they're so different in what they're suited for that it's hard to come up with a clear-cut winner.
I need an honest opinion as to which sounds better.
