Paul, I used to work for Stoner here in Houston. I have also been a client of theirs at The Williams Companies, Enron, and Shell. I have used both steady state and transient software from Stoner Associates, Gregg Engineering, SSI, and LiqConsult and can say that Stoner's is easily the most rigorous. I have example after example of it's better (although not always by much) results. It is also the most versatile and powerful.
Typically, the only real consideration is cost. You can rent or buy any of these software packages, but they are typically expensive.
I know this is a really late response, but I just found this forum 5 minutes ago!