Stardust potion gives you about 1.5k stardust per pot, while greater stardust gives you about 7.5k to 10k i think
The way to use smelting upgrade pot optimally is in crafting wiki
Stardust Potion-75xp Energy Potion-50xp Anti-Disease Potion-250xp Tree Speed Potion-525xp Smelting Upgrade Potion-550xp Great Stardust Potion-1925xp Farming Speed Potion-500xp Rare Monster Potion-2125xp