Dave,
I wish I could add something about the sodium citrate vs citric acid, but I cannot. I like others just do a net search and come to conclusions. So as a guess and it is just a guess, is that the reaction of sodium carbonate with citric acid forms multiple bonds. Just a google search, but I think that these multiple bonds could help prevent cheese sauce from separating. An outlet that may help is Dave Arnold or Harold McGee. All I know is that I have never had a failure using the sodium citrate. I purchased a large container from Willpowder.com as they had the best price. What I can add is that I do cut down on the amount of salt I add as with the cheese and the other items, the full formulation comes off as a little salty for our tastes. I have used both milk, beer, wine, water for the base and they have all turned out just fine. Good luck.
Ricky