Custom matcher via extension method vs. base class

CustomMatchersExample.cs

/* Simplified custom matcher example. 
   This shows matching args less than a specified int value. 
   More common use would be checking a few properties on a complex type (e.g. Car make, model, year)
 */

//Arg.Matches.LessThan(2);

public static class Extensions {
  public static int LessThan(this Match match, int number) {
    return match.WhenNoErrorsFrom(x => {
      if (x < number) yield return "should be less than " + number; // << -- won't compile due to yield
  }
}

// ==============
//      vs.
// ==============

//Arg.Matches(new LessThanMatcher(2));

public class LessThanMatcher : CustomMatcher<int> {
  int number;
  public LessThanMatcher(int number) {
    this.number = number;
  }
  public override IEnumerable<string> ReasonsMatchFailedFor(object arg) {
    var x = (int) arg;
    if (x < number) yield return "should be less than " + number;
  }
}

public static class Extensions {
  public static int LessThan(this Match match, int number) {
    // why yield? it's not like you need to be lazy here, right?
    return match.WhenNoErrorsFrom(x => x < number ? new string[] { "should be less than " + number } : new string[0]);
  }
}

@OJ,

If you are comparing multiple fields then laziness is nice, as you can have a IsSatisfied method than just checks for Any(), whereas getting the description of the errors can do a complete enumeration.

Not a big deal, just would be nice if I could throw yield in there. Will probably end up doing something like match.AddError("blahblah"), so caller's experience is very similar.