Dictionary

Question

I have a little bit confusion b/w Dictionary and Array.When should we use the Dictionary.
 I have a query that " I have some substrings  like 'of' , 'on , 'at' , 'above', 'near' and   so on.  I want  to check on each entery of a another string that  this  string contain any above  substrings  or not   .Then what could i use a Array or a Dictionary to store these substrings"

Exa:



  Dictionary<int, string> Substring = new Dictionary<int, string>();
  Substring.Add(1, "of");
  Substring.Add(2, "on");
  Substring.Add(3, "obove");
  Substring.Add(4, "at");
  Substring.Add(5, "off");

  string[] Substring_ = new String[5];
  Substring_[0] = "of";
  Substring_[1] = "on";
  Substring_[2] = "above";
  Substring_[3] = "at";
  Substring_[5] = "off";


  string Check = "of";
  int Find=0;
  //Method 1 By using a ARRAY//
  foreach(string  cnt in Substring_)
  {
  if(Check==cnt)
  {
  Find=1;
  break;
  }
  }


  //Method 2 By using a Dictionary//
  Dictionary<int, string>.ValueCollection Val = Substring.Values;
  foreach (string cnt in Val)
  {
  if (Check == cnt)
  {
  Find = 1;
  break;
  }

which method is good for point of view of complexity.