try/catch + using, right syntax

Since a using block is just a syntax simplification of a try/finally (MSDN), personally I'd go with the following, though I doubt it's significantly different than your second option:

MyClass myObject = null;

try
{
    myObject = new MyClass();
    //important stuff
}
catch (Exception ex)
{
    //handle exception
}
finally
{
    if (myObject is IDisposable)
    {
        myObject.Dispose();
    }
}

I prefer the second one. May as well trap errors relating to the creation of the object as well.