you can base it on price atm. eg i just checked yesterday on my server and copper bars/ore were 2g per stack, tin bars 2g/ore per stack. but bronze bars were 1g per stack. and i just needed 20 bronze, not 40, so i picked the bronze instead of making my own.
so while bronze bars are not in as high demand, it certainly sells lower than the coppers and tins together.
as for when you're selling, given that the ore and bar price is about the same, i'd suggest to go with ore. in ore form, any miner can smelt it (for his engineer or BS, which may be the same person). additionally, as ore, jewelcrafters can make use of it. they can prospect it as ore or smelt it to be bars, whichever way they need it.