Like Richard I think it much depends on the Swim Neil. Some swims naturally favour one or the other.
some swims that are pre cut on club or syndicate stretches have been positioned solely with a downstream cast in mind, such as having large overhanging bushes or trees on the upstream cast, making an upstream cast difficult or even impossible.
There are times when I choose a swim regardless of whether I can comfortably cast up or down, because the upstream cast looks so good, and of course vice versa.
Most often I do use two rods, but I do like to have a good distance between my baits, it's pretty pointless in my opinion having both baits only a few yards apart, if the swim is too tight to cast two baits, then I'll opt for one rod, but the single spot has to be very appealing or at least have form, where I'm happy enough to fish one rod.
I'm always looking at all options, up, down and both, there are for me two advantages for considering the upstream cast, that is often the Barbel aren't used to getting hooked in these areas, even though the upstream spot is not very far from the danger spots downstream it can still be a major advantage.
secondly there usually is no need for back leading, because the flow even a very gentle one will pin you line down for you, the possible exception being when there is a stiff flow you're casting much further out into the flow.
But having said all that I would guess more than 90% of my Barbel fishing involves a downstream, and upstream cast, only very occasionally will i opt for one or the other