Take a look:
Quinoa nutrition facts
Baked potato nutrition facts
Boiled potato nutrition facts
I don't care about complete proteins from my carb sources, just the EAAs. And potatoes and quinoa are both equal on that front. Quinoa has more protein per serving, but are you choosing a protein source here or a carb source? Do you eat lamb because it has a higher amount of carbs per pound than chicken? No? Then don't choose a carb based on protein. That said, it is a good idea to pay attention to the EAAs in your carbs, because proteins and aminos compete for absorption in the small intestine, and you definitely want a higher percentage of EAAs than NEAAs in your food.
*Do* choose a carb based on vitamins and minerals.
Potatoes are listed as having a higher completeness score for vitamins and minerals than quinoa. Mostly because quinoa doesn't have Jack Squat for vitamin C. But more importantly, what do you know about potassium? How much do you need a day? (A: 3.5 grams-ish). How much do you get from your diet? (A: probably half that or less, seriously). What is potassium used for? (A: nerve impulses for muscle contractions... something important to most of us on these forums, no?). And you know what else is cool about potassium? Unlike sodium, which is mainly in the fluids between cells and in the blood, potassium is mostly _in_ the cells, especially muscle cells. The upshot of that is that a higher concentration of potassium in the muscle cells will pull water into the cells, keeping them more hydrated, and making your muscles look fuller. Now compare quinoa to potatoes as a potassium source. Yeah, spuds rule when it comes to potassium, especially baked potatoes (or mashed).
Devils advocate: Quinoa is a better source of other minerals, like phosphorous, zinc and manganese, than potatoes. Now, if you're taking a multivitamin it's a moot point, otherwise quinoa wins as a source of micro minerals.
Ok, to answer your question more simply: Are potatoes a no go? To me, they're a definite YES. Potatoes and a multivitamin trumps quinoa on price, and has all the minor minerals, plus lots of potassium. And there's the taste. Quinoa isn't horrible or anything, but a big baked potato with your daily allowance of saturated fats (e.g. butter and sour cream) beats quinoa hands down in my opinion.