Nice.
I dislike that, since there's already a unique sling with that property. But regardless, I have no real objections against it since in my games, whoever wields a sling has a low strenght (druids, mages etc.)
Dagger vs. Short Sword vs. Long Sword:
Imo, Long swords are a great choice not because of their damage, but because of their properties/equiped effects. There really (imo) isn't a rivarly in between long/short sword for my warriors. You got flaming, level draining, immunities, vs undead, vs dragons, elemental damage.....it's the sheer amount of options which makes them great, and which short swords lack (not that short swords are bad, but they're more thief-oriented).
Daggers - you can throw them, which is a nice boon in BG1 (use 1 proficency for both melee and ranged weapon).