During a takeoff emergency, is it safer to abort or continue the takeoff if either can be done within the available runway?
In my experience, admittedly dated (retired 1999) whether it's 'better' or not didn't enter into the consideration. A reduced-power assumed-temperature was always used (unless there was something that prohibited it) because of the cost factor, and the power reduction used was always such that there was the proper amount of runway but not 'plenty' of runway. There was a maximum amount of reduction above which you could not legally go, but that was rarely used in my experience.

All that said, it's the captain's airplane. While I was a 747 f.o., I occasionally saw captains use less than the power reduction called for. More often, but still not a lot, I saw captains simply call for a max power takeoff rather fussing with adjusting the reduced power. Max power takeoffs were always entered into the aircraft maintenance log because it was required to do one every so often. I forget the exact requirement..

When I was a captain, I almost always observed the recommended reduced power. Twice doing this it became a closer thing than I would have liked. The first time was on the reef runway at Honolulu. We were over 800,000 lbs, it was my leg, and we lifted off at the very end. I rotated a couple of knots or so early because the end seemed to be getting very close. I thought a lot about that takeoff and discussed it with a number of people. My view, and I think the consensus view was: beware of high-humidity reduced power takeoffs. Humid air is less dense than dry air, but the performance charts (at least the ones we used) didn't take that into account.

A year or so later at Dhahran, Saudi Arabia, we were bringing a full load of troops back from Desert Storm. Reduced power was called for, the humidity was very high, and it was the f.o.s leg. A little over half way down the runway I didn't like how it was looking, and called for max power. Discussing it later with the f.o. and the f.e., we had all come to the same conclusion at about the same time. I had simply been the first to verbalize.