Frozen veggies would always be more nutritious than canned. Canned veggies have already been cooked at high temps and lost many nutrients then have additives added such as the salt mentioned. Frozen veggies appear to have there nutrients locked in and available when ready for consumption but preperation such as boiling will leach the vitamins into the water which most times is not consumed and you have lost out there. Raw veggies would always give you the most vitamin and nutrient rich amount.
Another side would be Frozen vs. Farm fresh.
If you don't get around to eating your fresh foods they will lose vitamins and nutrients after just days in your fridge so you will need to buy small amounts and replenish often. Frozen veggies have them locked in and waiting. Down fall to frozen is mineral content which is greater in a small market veggie. Small market local growers have the advantage of rich mineral soil over larger market growers.
My vote would be frozen or fresh but in the raw form for best vitamin and mineral consumption.