- An unenchanted dagger gives you an attack bonus of +0.5 (+0 intrinsic
and +0.5 excess strength bonus), for a total of 3.5-4.5 damage.
- Enchanting the dagger brings it up to +1.75 (+1 intrinsic and +0.75
excss strength), for a total of 4.75-5.75 damage.
- Unenchanted average: 4; enchanted average: 5.25. Damage improvement: 32%.
- Enchanting your weapon would also give you +5 to accuracy1, increasing it from 75 to 80. I consider this negligible.
32% more damage means you'll be able to kill monsters in 32% less time, meaning you'll take 32% less damage from them in melee. How does this fare against the leather armor?
- When unenchanted, the leather armor gives you +3.5 protection (+3 intrinsic and +0.5 excess strength bonus).
- When enchanted, the leather armor gives you +4.75 protection (+4 intrinsic and +0.75 excess strength bonus).
- Unenchanted: +3.5. Enchanted: +4.75. Armor improvement: 35%. The wiki says "Every point of armor gives you another 13% chance to dodge a monster's attacks."1 So it improves your dodging chance from 45.5% to 61.8%.
In principle, it seems roughly as beneficial to enchant either the dagger or the armor, because you'll end up taking 32-35% less damage with either (because, as Ids said, killing monsters faster also means they'll have less time to damage you). Enchanting the armor would also protect you against arrow turrets and non-magical ranged attacks, while enchanting the dagger would give you a better chance against armored and/or regenerating monsters. Choose based on what kills you more often.
I would personally go for the darts, though.
- Unenchanted darts give you an attack bonus of +0.5 (+0 intrinsic
and +0.5 excess strength bonus), for a total of 2.5-4.5 damage.
- Enchanted darts give you +1.75 (+1 intrinsic and +0.75
excss strength), for a total of 3.75-5.75 damage.
- Unenchanted average: 3.5; enchanted average: 4.75. Damage improvement: 35%.
The numbers show that you gain as much damage as if you enchanted the dagger, so stats are not an issue. Obviously the big downside of darts is that you have a limited supply of them, and the big advantage is that unless you're lucky to find a staff of firebolt, lightning or poison early on, the darts will be your only ranged weapon for the entire early game.
Here's my take. Having limited darts is not as bad if they're enchanted, because since they'll do 35% more damage (and they'll be 5% more accurate), you won't be wasting as many of them on monkeys and the like. You'll be able to save them for pit bloats, acid mounds, and other nasty monsters that you have no reliable way of dealing with in melee. Enchanting your dagger or armor will make most of the early game a little easier, but enchanting your darts might save your life.
It's also a lot harder to find more darts than it is to find better weapons and armor, such as the scale mail (12 str, +4 armor) or the spear (13 str, 4-5 damage). So darts it is.
Ids brings up a great point, though: you can now identify scrolls of enchantment. Therefore, you should not only think of what you'll do with the one you just used, but also think about how you'll use the ones you're bound to find later. You will have a better chance of enchanting more powerful items than the dagger or leather armor, or perhaps a staff of firebolt, so chances are that you won't keep the armor or the dagger for long anyway.