I think you can judge by your own way, by examining your portal requirement.
My meaning is that you need to chose .Net framework based on your requirement. if you want to use latest functionality of .net framework then definitely 4.0 is best for your portal. But while making it online than you need to pay more for 4.0 framework support web server then 3.5 or 2.0.
so please be more specific with your requirement. and then choose better option for your framework selection for your portal.
3.5[
^]
4.5[
^]
4.0[
^]