MentalRay Vs Renderman Shaders

So I get asked quite often why Renderman shaders can be a pain in the ass to use compared to Mentalrays shaders. My standard response is that Mentalray shaders are not as art directable compared to Renderman shaders. Using an analogy, one can compare mentalray as canned foods, while renderman are you veggies, meats, etc., in essense, basic ingredients. You can prepare a meal both ways, both can be delicious, but you can't get variety with canned foods but it is fast. While using the basic ingredients you can cook anything up, but it takes more skill and more time consuming. So, it really depends on the companies needs, some projects will require a unique look, while some require speed.