Here is my perspective when shopping for suspension parts. Coming from a motorcycle perspective, where suspension has developed to the point that you nearly can't break a motorcycle, I want suspension components that are just as tough for my 4Runner. Or at least, as tough as practicable and affordable.
I look at the construction, materials, dimensions and coatings. The Donahoe coil-over shock offers a full 2.5" diameter with a thick shaft coated with a very durable coating. To my way of reason this is a big motorcycle shock. The Baja racing heritage of Donahoe shows that their products are designed for severe use.
I have seen and held the Old Man Emu components in my hands. There are an improvement over the OME Toyota offers. But, it is not nearly the product Donahoe offers.
If you are looking for a quality product to serve you well, then I'm sure the OME will get you there and back. If you are looking for suspension parts that are a definite cut above, and able to deal with the most demanding use, then the Donahoe products are the way to go.
I don't know if there is all that much of a cost difference between the options, once labor is factored into the equation. Certainly spacers offer the look without any substance, at a "bargain" price. The OME products are quality products (I'm wild about their protection products) at a nearly racing product price. Then there is Donahoe, at the top of the heap in terms of price (still inexpensive compared to one-off race goods).
I would suggest that there is more profit made for the manufacturer of the spacer lift products than the other two. Holding both the OME and Donahoe products and looking closely at them, I'd say that the Donahoes are an exceptional value.
For me, I think of the aftermarket offerings for the 4Runner as delights on the buffet. I pick and choose what I consider to be the most tasty form across the entire menu. Just because I am enthusiastic about the ARB protection offerings doesn't mean that I will ignore other products where I think they offer more.
But then, I use my 4Runner off-road as part of my work. I also spend untold hours alone in the middle of public lands. I rely on my 4Runner, and all of the aftermarket products I use, to get me home. Many buy SUVs to look good at the mall. They can call AAA if something breaks, I can't.