Which is the better choice, grass fed ground beef or ground turkey thighs?

Turkey on both counts. Ground turkey instead of ground beef means only the equivalent of a few percent fat whereas beef runs up to 20% fat or more. You have to use oil when pan frying turkey burgers since they do not have enough fat to keep them from sticking, unlike beef which has plenty of fat to keep a burger from sticking. I chop green olives and shallots into small pieces, to mix in by hand with the ground turkey when I make patties. I also add a pinch or two of chili pepper and about a tablespoon of Dijon mustard per pound of ground turkey. Last thing, a turkey burger runs about half to two-thirds of the calories of the same sized ground beef burger and less than one-fourth of the fat.

Ground turkey is leaner and generally better for you, although if you buy a good quality ground beef it isn't bad for you either. My boss always told us that grain-fed cows tasted better than grass-fed cows, though.

Turkey has less calories and fat than beef does

The grass the cow ate.

But it's not very tasty.

You need to read your lables, their are a lot of variables here. If you talking skinless thighs verses 95% lean probably the turkey.