Frozen broccoli is easy and cheap. Frozen spinach is cheep and good in smoothies. Canned stuff tends to not only taste kind of gross, but they line the cans with BPA(which is a hormone disruptor) to make sure the food doesn't corrode the metal of the can. Canned soup is one of the worst offenders. Then again frozen fruits and veggies aren't open to the air for very long before being preserved ...which means the pesticides used on them haven't had a chance to decay and are frozen right along with the food.
Pick fresh. But if your ultimate choice is between frozen or canned, frozen veggies will likely retain more nutrients.