Raw ore of base metals appeal to jewelcrafters for prospecting and can be used by other crafters (such as Blacksmiths and Engineers) if they have Mining to a sufficient level for smelting the ore.
Bars appeal to all blacksmiths and engineers, especially if they don't have Mining (or haven't skilled up sufficiently yet) or if they're short on time (or lazy...). Some bars will also appeal to jewelcrafters who are not miners, for example when doing the 1-300 levelling of jewelcrafting you need bars for items like Mithril Filigree and Thorium Setting. I'd suggest going half and half until you get a feeling for demand.
Dark Iron Ore requires special consideration. It's used in two quests, one is the repeatable reputation quest for Thorium Brotherhood and the other is the warlock epic mount quest. It's also a pain to smelt (requires a trip to BRD), takes 8 ore for 1 bar and most of the items that bars are used for are obsolete. Personally I would always sell ore, not bars.
Note that uncommon metals (silver, gold, truesilver and eternium) cannot be prospected and isn't used in any quests. It may as well always be sold in bars.