They are both great - top-notch greens to choose.
If you're a woman, Spinach is great to boost your intake of folate. Folic acid is one of those necessary dietary requirements for prenatal development as well as to prevent cervical cancer. Many women don't get enough dietary folate. Spinach is higher in folate and vitamin E than Kale.
I've always heard spinach is better, but http://www.nutritiondata.com says Kale has more vitamins A, C, and K and is slightly, slightly higher in other vitamins and minerals (except folate and vit. E) and protein, but also slightly higher sodium & calorie count. Consider the source, it looks legit. but sometimes sites like these aren't.
Something to consider with both of these is that vitamin K promotes blood clotting. If you're taking blood thinners, the vitamin K in these and other dark leafy greens can counteract the effects - so talk to the doc. that prescribes your anticoagulants about plans to increase leafy greens in your diet, if you're taking or need those medications.
Great choices for a green smoothie! Both of them are high-nutrition greens. Can't say which one is best for you, that depends on the rest of your diet and what you need. Either way, since both are good for different reasons I wouldn't binge and purge but rather alternate them in your smoothies every 2 days.