char.IsDigit vs Expression

using System;
using System.Text;
using System.Diagnostics;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var stopwatch = Stopwatch.StartNew();
            const int times = 100000000;
            for (int i = 0; i < times; i++)
            {
                var output = RemoveNonNumbers1("oa9sd01234567897fa9");
            }
            stopwatch.Stop();
            Console.WriteLine(stopwatch.Elapsed);
            stopwatch.Restart();
            for (int i = 0; i < times; i++)
            {
                var output = RemoveNonNumbers2("oa9sd01234567897fa9");
            }
            stopwatch.Stop();
            Console.WriteLine(stopwatch.Elapsed);
            Console.ReadLine();
        }

        internal static string RemoveNonNumbers1(string p)
        {
            if (string.IsNullOrEmpty(p))
                return p;

            var sb = new StringBuilder();
            foreach (var c in p)
            {
                if (c >= 0x30 && c <= 0x39)
                    sb.Append(c);
            }
            return sb.ToString();
        }

        internal static string RemoveNonNumbers2(string p)
        {
            if (string.IsNullOrEmpty(p))
                return p;

            var sb = new StringBuilder();
            foreach (var c in p)
            {
                if (char.IsDigit(c))
                    sb.Append(c);
            }
            return sb.ToString();
        }
    }
}