StringBuilder vs String.Concat

// In response to: http://trelford.com/blog/post/StringBuilder.aspx

open System
open System.Text

let random, sb, n = new Random(), new StringBuilder(), 100000000

let inline getstring length =
    new String(Array.init length (fun _ -> random.Next(97, 123) |> char))

#time "on"

for i = 1 to n do
    String.Concat(getstring(6), getstring(6), getstring(6), getstring(6))
    |> ignore

#time "on"

for i = 1 to n do
    sb.Length <- 0
    sb.Append(getstring(6)).Append(getstring(6)).Append(getstring(6)).Append(getstring(6)).ToString()
    |> ignore
;;

/*
ITERATIONS: 100
    CONCAT
    Real: 00:00:00.001, CPU: 00:00:00.000, GC gen0: 0, gen1: 0, gen2: 0
    val it : unit = ()

    STRINGBUILDER
    Real: 00:00:00.001, CPU: 00:00:00.000, GC gen0: 0, gen1: 0, gen2: 0
    val it : unit = ()

ITERATIONS: 1000
    CONCAT
    Real: 00:00:00.001, CPU: 00:00:00.000, GC gen0: 0, gen1: 0, gen2: 0
    val it : unit = ()

    STRINGBUILDER
    Real: 00:00:00.001, CPU: 00:00:00.015, GC gen0: 0, gen1: 0, gen2: 0
    val it : unit = ()

ITERATIONS: 100'000
    CONCAT
    Real: 00:00:00.078, CPU: 00:00:00.078, GC gen0: 10, gen1: 3, gen2: 1
    val it : unit = ()

    STRINGBUILDER
    Real: 00:00:00.045, CPU: 00:00:00.046, GC gen0: 9, gen1: 1, gen2: 0
    val it : unit = ()

ITERATIONS: 1'000'000
    CONCAT
    Real: 00:00:00.430, CPU: 00:00:00.436, GC gen0: 95, gen1: 3, gen2: 1
    val it : unit = ()

    STRINGBUILDER
    Real: 00:00:00.413, CPU: 00:00:00.421, GC gen0: 95, gen1: 1, gen2: 0
    val it : unit = ()

ITERATIONS: 100'000'000
    CONCAT
    Real: 00:00:39.005, CPU: 00:00:39.000, GC gen0: 9537, gen1: 6, gen2: 1
    val it : unit = ()

    STRINGBUILDER
    Real: 00:00:40.887, CPU: 00:00:40.841, GC gen0: 9537, gen1: 5, gen2: 0
    val it : unit = ()
*/