Talli,
One would have to know a lot about the particulars of their environment to determine which is the better choice.
Technology choices need to be viewed as business decisions and one of the key business questions is sustainability. It's one thing to build an application as a one-off. It's quite another to keep it evolving in response to user needs and new markets.
If I were in their shoes, some of the questions I would ask: How much does this application need to evolve? Do I have .NET expertise in house and the budget to continue to enhance the application?
OpenACS has an inherent advantage over .NET and that's the *community* of very smart people supporting and developing the toolkit. For us SloanSpace/.LRN has been a huge success. Could we have done it in J2EE or .NET? Sure. It would have cost us a hell of a lot more. More importantly, we would not be in a position to leverage the ongoing work and innovation of the community. That's the beauty of open source.
