The 18-23c tube is exactly the size you want to use with a 25c tire. In my book the 25-35c tube is TOO big. The tube will stretch easily to size. Try pumping up a tube free from a tire and see how far it will expand (don't go overboard though).

When fitting a tube to a tire its MUCH easier to have a small tube which expands rather than a larger tube which your trying to stuff into the space available. You are MUCH more prone to pinching a larger tube when fitting rather than a smaller one.

Anthony