The Instrument Rating is by far one of the best tools to have in a pilot's skill set. Giving you the ability to fly in less than ideal conditions it teaches you to read the weather like a meterologist, flight plan with a high degree of accuracy and is essential for becoming a professional pilot.
We recommend the Diamond DA-40 or Piper PA-28 for this rating. Typically you'd spend the first half of the syllabus in the DA-20 and then transisition to the 172 or the DA-40 for the more advance and cross country lessons.