% att versus % stat

+%atk is not the same as +atk.

To clarify: a good claw+stars has far less attack than a good sword or bow.
While a Hero would have about 200 w.att, an equally equipped NL would have only 150.

So, a +100 w.att apple would give the Hero 50% boost to his damage, but the NL 66% boost.

However, +21% attack would give them both, well, a 21% boost to their damage.
In absolute w.att, it would add 42 attack to the Hero, but only 31.5 to the NL.

Right but the comparison is not between +atk and +%atk or non-NLs and NLs, it's between +%atk and +%stat.

so is there a point where % att or % stat is more effective?

Use A as attack and S1 as primary stat (S2 for secondary if you're picky) and stick it into the damage formula after adjusting for the +%. If you set the +% value you would get two values with A and S1 in it. Compare the two in an inequality and isolate a variable. If no realistic values or all realistic values satisfy the inequality, then there is no point. If there is, that's the point (or rather a border).

If you plan on being technical, you're going to deal with five variables and multiple formulas for different jobs. Might be faster just sticking in your specific attack and stats w/ the potential equip on in the formula and see which one's better. A general equation is not worth finding (or useful).