Premium will give you best effect but also requires most materials to be used. You can craft ammo for Plasma Accelerator, Multi-Cannon, Cannon, Fragment Cannon, Railgun, Missiles, Mines. Basic ammo will give you 50% of full ammo load with no bonus while Standard will completely refill your ammo and give you 25% damage bonus, Premium will give you even more - 50% damage bonus (bonus will be tweaked down).
So basic ammo is pretty much useless because you waste materials for 50% of max ammo load unless you really need that ammo now, standard and premium are fine (even after devs reduce bonus to 10-15/20-25% or so instead of 25/50% that is now).
Probably smartest will be to have 1 Huge weapon (Cannon or Plasma) and have several loads for that weapon in case you need it, but as for Railguns - 31 rounds are just not enough, one Railgun is never effective and you need at least 2-4, so you need 1 reload of crafted ammo for each Railgun making it very ineffective in the end because 1 reload only gives 31 round (basic reload gives 16 rounds).
Will see, maybe Railguns will get some love or maybe finding materials for Railguns will be MUCH easier than for other weapons but as for now i don't see myself bothering with driving around to find 12 specific materials just for 31 round of 1 Medium Railgun when you can get 105 Huge Plasma/Cannon rounds in similar amount of time.