Regardless of what the price in DEC is: a fixed price DEC is 100% better player experience. If the in-game store can find a way to add an additional fixed DEC/credit surcharge to buy/exchange and burn the voucher, that will make the buying process soooo much easier for new players that just want to buy a pack with credits.
(Also why are we calling them GEMS in the store instead of RIFT Packs? Make it obvious for new players)