The NES version is much, much more difficult than the Dawn of Souls version. Dawn of Souls is really a mockery of the original. Don't get me wrong, the extras were nice, but the game was drastically changed, as opposed to the Origins version, where any additions that were made could be turned off.
Some Examples of Changes are:
Dashing - No dashing in the original.
Spell structure - (This is probably the biggest change, in the original there was no such thing as MP. There are eight levels of spells, and you can cast a certain amount of each level. Certainly made the game much more difficult, as you really had to watch what spells you were using. None of that Fire 2 every random battle. Unless they were the Ice Wolves. Then you'd FIR2, and run back to the inn to heal.)
Life 2 - (Couldn't be used in battle in the original)
Elemental/Monster Specific Weapons - (They didn't work in the original. Giant Sword wouldn't do more damage to Giants, and so on. More of a programming error than anything else)
Max Level - As I recall, the max level in the original was 50. It's been awhile though, so it might actually be lower.
Saving - You could only save at inns (Or Tents/Cabins) in the original. Die in the Ice Cave? Join the club, everyone does.
Buying Items - You could only buy in quantities of one. Turbo controlers made buying 99 potions a little less time consuming.
Ineffective - That's right. Another big change. Repeatedly hitting the A button? None of that in the past. If you had all four Light Warriors target a monster, and the first one killed it, your party members would not change their target. They'd swing at what you told them, and you'd be treated to a lovely Ineffective. Made battles a lot less boring, as you have to actually pay attention.
There are probably more changes, but I'm just going off the top of my head here. What I've listed might not look like much, but it alters the game significantly. Dawn of Souls is most definitely the easiest, and least true to the original.