C#：Guid.NewGuid()和DateTime.Now该选择哪个？？？

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("正在计算Guid.....");

            Stopwatch stopwatch = new Stopwatch();
            stopwatch.Start();

            for (int i = 0; i < 10000000; i++)
            {
                Guid guid = Guid.NewGuid();
            }
            stopwatch.Stop();

            TimeSpan t = stopwatch.Elapsed;

            string totaltime = String.Format("{0:00}:{1:00}:{2:00}:{3:00}", t.Hours, t.Minutes, t.Seconds, t.Milliseconds);
            Console.WriteLine("使用Guid时，总用时：{0}", totaltime);//2s

            //----------------------------------------------------------------------------------//

            Console.WriteLine("正在计算DateTime.....");
            stopwatch.Start();

            for (int i = 0; i < 10000000; i++)
            {
                DateTime dateTime = DateTime.Now;
            }
            stopwatch.Stop();

            TimeSpan t2 = stopwatch.Elapsed;

            string totaltime2 = String.Format("{0:00}:{1:00}:{2:00}:{3:00}", t2.Hours, t2.Minutes, t2.Seconds, t2.Milliseconds);
            Console.WriteLine("使用DateTime时，总用时：{0}", totaltime2);//3s

            Console.ReadKey();
        }
    }
}