Free choice
no measured difference
Swap raisins and chips however you like
Cooks who made the swap in both directions reported no consistent change in chew, spread or how the cookie set.
Chocolate chips and raisins are close enough in size and in how they sit in the dough that the cookie around them does not notice. Cranberries behave the same way, and leaving the nuts out changes the bite where the nuts were and nothing else. This is one of the few decisions in baking that really is just preference, so spend it on what you want to eat.
We looked for a difference: this was written down as a prediction before the reviews were read, and the reviews agreed. A null result we expected is still a result.
This one is real
Soaking the raisins puts water in the dough
Twenty minutes in hot water turns raisins into little reservoirs, and they carry that water into the bake with them.
One cook got a better edge from it. The most complete test of it got a better centre and a WORSE structural result from the same change, which is exactly what you would expect from adding water to a dough whose oats are already competing for what is there. Plump raisins are genuinely nicer to eat. They are not free.
If you soak: drain them properly and pat them dry, and treat the dough as slightly wetter than the recipe intended rather than adding more liquid on top.
Worth knowing
Add them at the end, whatever they are
Inclusions go in last, once the flour is barely combined, for the same reason in every cookie.
Mixing past that point works the dough rather than distributing the fruit, and this dough has enough going on structurally without it. Stir them through by hand if the mixer has done its job already.