There's no reason you can't ask on both sites - simply state you've asked in both places. (This has happened several times that I can recall) At worst you'll get two different audiences giving you possible answers. However as Jeff pointed out - try one first and if you get a lukewarm response, then try the other site.
If you think it's something a .NET dev would be able to answer - ask on StackOverflow first but there is a lot more overlap between Server Fault and Stack Overflow than you would think.