Well, it depends, really. If there is an expensive lookup that takes a lot of processing power, it pays to have that formula in a helper column and refer other formulae to that result instead of repeating the same lookup over and over again in a single, long formula.

In other cases, you won't achieve any benefits from splitting your calculation into separate cells, so you may as well do it all in one cell.

It often also depends on the proficiency of the user and the level of formula nesting they can stomach. Long formulae tend to be unwieldy and hard to troubleshoot, especially if you have not written them yourself or if you have written them a while ago forgotten what you were after.

But with tools like Evaluate Formula, it's not too hard to figure out what a formula does.

Speed of calculation would be the most important factor, especially with large datasets, and maintainability the other.

There are almost always alternatives to a solution that suit one person better than another.