Difference between ( ) (parenthesis) and { } (curly brackets) in Razor

If you're familiar with WebForms, it's very similar to the difference between <%= %> (or <%: %>) and <% %>. The former is evaluated then outputted to the page, the latter is a block of code that can do whatever it needs (but isn't written to the page).

The paren is just an explicit expression, and you will notice that you do not need a semi-colon. The brackets are a code block, to be used just like any other piece of code. Expressions have their output submitted as part of the HTML, whereas code blocks do not.

Phil Haack actually wrote a good primer on some Razor syntax

To the question: "why is the second one not valid?", in addition to what Betty and Justin say, the issues specific to what you show: inside curly braces you need your code to follow the normal syntax of c#, so you can't have just a loose "a==b?c:d", without assigning the result to something. And you need a semicolon. So you could say

@{string message = Model.WillAttend == true ? 
     "This will be an exciting party with you" : 
     "So sorry. You'll lose the beeer";}