Combine a boolean condition with a Where clause in same query
Taking @VisualMelon's advice,

I converted it to this code, also replacing the compare with equal, maybe better.

List&lt;KeyValuePair&lt;int,string&gt;&gt; Groups = 
model.Groups.Where(
  m =&gt; string.Equals(m.Value, 
                     "properties", 
                     StringComparison.OrdinalIgnoreCase) == isProperty)
  .ToList();