Re: Seymour Duncan vs. Dimarzio
As a trend but NOT an absolute, to my ear, DiMarzios rely heavily on the midrange spectrum whereas the Duncans rely on the high and low spectrum for their signature sounds. This comes out especially in each's single coil offerings. DiMarzios are sweeter but Duncans are crisper.
DiMarzio appears to want their pickups to have a predictable, predetermined sound, regardless of which guitar they are installed in whereas Duncan appears to want the wood/hardware of the individual guitar to factor in.
In terms of full sized humbuckers, I think DiMarzio has a 'perfect' bridge pickup for just about anyone's taste whereas Duncan does not. Conversely, I think Duncan has a 'perfect' neck pickup for just about anyone's taste whereas DiMarzio does not.
For stacks, I believe the absolute best are Kinmans where DiMarzios are about 95% as good. I don't like Duncans at all. To me, they all sound the same with varying volumes.
For acoustics, I honestly don't think anyone has gotten this down yet, not even Fishman. I still believe nothing beats a "high end" ball mic.
For actives, I think the newer Duncans MIGHT be starting to chip away at EMG's monopoly pedestal.
As to quality, there's no issues on either part: DiMarzio seems to rely on modern manufacturing techniques whereas Duncan seems to embrace more time honored ones.
On a slight tangent: When I was in high school, there was this one guy who was a Jimmy Page fanatic. To this guy, EVERYONE sucked except Page. One day, someone said to him that it would be most depressing if what he said was true because no matter which concert anyone went to, be it Hendrix, Clapton, Beck (Jeff, that is), and on and on, all would suck except for Zeppelin ones and down the road, by necessity, he'd have to get bored at the same Jimmy Page stuff over and over again because there simply wasn't anyone else (per him) that was or would ever be worth listening to. The guy put on a somewhat depressed face and after a moment of thinking, agreed. The point here being: More than one brand of pickups can and will get the job done and one shouldn't lock his or herself into one brand as it cuts off SO many sonic opportunities. I, myself use a DiMarzio Virtual Hot PAF as my #1 bridge pickup and a Duncan Pearly Gates bridge model pickup as my #1 neck pickup. To me, this is the absolute best vintage combination for someone wanting both balls and bells. As to the whole loyalty to one manufacturer thing, when any manufacturer systematically gives you their products for free, you might want to consider making some concessions. Aside from this, it is not illegal or immoral to mix pickup brands if this is what works best for you.
My disclaimer: This is only MY opinion, YMMV.