test 1

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net;

namespace ConsoleApplication3
{
	class Program
	{
		private static string hoge;
		private static string fuga;

		static void Main(string[] args)
		{
			Test();

			Console.ReadKey();
		}

		private async static void Test()
		{
			TestAsync2();
			await TestAsync();
		}

		private async static Task TestAsync()
		{
			var startTime = DateTime.Now;
			await Task.Factory.StartNew(() =>
				{
					for (int i = 0; i < 100000; i++)
					{
						hoge += "f";
					}
				});

			Console.WriteLine(DateTime.Now.ToString("HH:mm:ss.fff") + " TestAsync Time: " + (DateTime.Now - startTime).TotalMilliseconds + "msec");
		}


		private async static void TestAsync2()
		{
			var startTime = DateTime.Now;
			await Task.Factory.StartNew(() =>
				{
					for (int i = 0; i < 100000; i++)
					{
						fuga += "f";
					}
				});

			Console.WriteLine(DateTime.Now.ToString("HH:mm:ss.fff") + " TestAsync2 Time: " + (DateTime.Now - startTime).TotalMilliseconds + "msec");
		}
	}
}

test 1 result

10:43:54.464 TestAsync2 Time: 2531.3014msec
10:43.54.464 TestAsync Time: 2515.6764msec

test 2

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net;

namespace ConsoleApplication3
{
	class Program
	{
		private static string hoge;
		private static string fuga;

		static void Main(string[] args)
		{
			Test();

			Console.ReadKey();
		}

		private async static void Test()
		{
			await TestAsync2();
			await TestAsync();
		}

		private async static Task TestAsync()
		{
			var startTime = DateTime.Now;
			await Task.Factory.StartNew(() =>
				{
					for (int i = 0; i < 100000; i++)
					{
						hoge += "f";
					}
				});

			Console.WriteLine(DateTime.Now.ToString("HH:mm:ss.fff") + " TestAsync Time: " + (DateTime.Now - startTime).TotalMilliseconds + "msec");
		}


		private async static Task TestAsync2()
		{
			var startTime = DateTime.Now;
			await Task.Factory.StartNew(() =>
				{
					for (int i = 0; i < 100000; i++)
					{
						fuga += "f";
					}
				});

			Console.WriteLine(DateTime.Now.ToString("HH:mm:ss.fff") + " TestAsync2 Time: " + (DateTime.Now - startTime).TotalMilliseconds + "msec");
		}
	}
}

test 2 result

10:42:26.737 TestAsync2 Time: 1421.8846msec
10:42.28.581 TestAsync Time: 1828.1737msec