All or nothing.....
I choose to use both so that I can minimize the total amount of nutrient input.
Most plants have a nutrient demand curve that spikes up during certain parts of the season and also subsides during other times. It is nearly impossible to match this curve with synthetics because they release and disappear too rapidly. It is also hard to follow the curve with just organics because they release slowly and one would have to apply an abundance to meet peak demand which is costly and excess during times of low nutrient demand.
Now imagine you take an average of that curve over the growing season and we can simply call that "baseline fertility". It really is way more scientific than that but I would like to include everyone in this conversation not just the holier than tho.
I believe the baseline fertility needs of a plant are best served organically because its stable, and gets the soil active. But mostly it means that you can apply organic applications at the same cost as synthetic ones because the rates are lower.
The next step is to hit the growth spikes with a low rate of synthetic fertilizer. The synthetic nutrients are able to be rapidly released and absorbed by the plant during these narrow windows of peak nutrient demand.
So in the end, you use a 1/2 rate of organic fertilizer and a 1/4 rate of synthetic fertilizer to accomplish optimum growth with less nutrients and money.