A full scale abort on a limited length runway is probably the most critical decision that any captain will make in a lifetime. It maybe a matter of life or death for a lot of people. A conversion course in a flight simulator does not necessarily prepare you for this decision because it is not the real thing. Basic abort skill - yes. But not the gut tearing decision that could cost or save lives.
While it might be a warm fuzzy feeling to completely trust the first officer to make a splendid well handled abort while the captain takes over at 70 knots, the widely varied past experience level of first officers from 500 hours TT to thousands of hours RH seat, makes it difficult for the captain to judge if he should do the abort himself or abbrogate that responsibility to his second in command.
There is probably no one right answer. For my part, it is no contest. The captain makes the abort decision and immediately takes over full handling control of the abort. The first officer takes up a support and monitoring role.