Well, you choose the one that yields the best result when you take into account money spent plus the time it takes to earn that money. So if I can make money at 100k/hr, I would train in a way that costs me 500k and can be done in 3 hours (8 hours total) rather than something that costs me 1m and can be done in 1 hour (11 hours total). Most people instead pick the second option since they see twice the cost for less than half the time.
You also need to take into account byproducts, how much you will actually get rid of instead of sell to save time. For example, flyfishing and dropping the fish is one of the fastest ways to train fishing, it's relatively low cost but you make no profit. It would be possible to bank the fish, but when you take into account the act of banking it would be more efficient to fish a different kind of fish that takes longer to catch but gives you more exp per catch.
For smithing this could be irrelevant, seeing as you would need to bank to get more supply, but you could also take into account the fact that it would be less profitable to smith gear at the anvil near the varrock east bank, yet faster than smelting bars in falador. IMO if you want to have it midway you need to find some kind of gear that roughly equates to the price of the raw material used to make it, then smith it in varrock. If you're up for it you could mine your own ores, saving you a ton of money and training mining, but probably more than doubling your work time.
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