aggregate.cs

var sw = new System.Diagnostics.Stopwatch();
var x = new []{"foo","bar","baz"};

sw.Start();
foreach(var iterations in Enumerable.Range(1,10000000))
{
    var result = x.Aggregate ((a,s)=>a+ ", " + s);
}
sw.ElapsedMilliseconds.Dump();

stringjoin.cs

var sw = new System.Diagnostics.Stopwatch();
var x = new []{"foo","bar","baz"};

sw.Start();
foreach(var iterations in Enumerable.Range(1,10000000))
{
    var result = string.Join(", ",x);
}
sw.ElapsedMilliseconds.Dump();