As chul said, there isn't a single answer. I would add that
- Vb.Net currently works in every corner of the product (Member Formulas, Complex Expressions, etc), so standardizing on that will likely pay dividends in unexpected places right away; whereas C# is limited to Business Rules namespaces at the moment, and it might be a while before it gets in all places where VB is.
- VB also tends to be easier for non-IT folks (accountants etc) to understand.
- C# is definitely more popular in the IT world, so recruiting for integration roles will probably be easier with that; whereas VB is more popular in the traditional EPM world, so recruiting for more finance-oriented roles will probably be easier with VB.
- The lifetime of a Onestream application can be hard to predict; a business will probably want it working YESTERDAY, so whichever language makes you more productive right away is more likely to result in immediate success, with all that entails. Premature optimization is the root of all evils, Knuth would say...