Next Step: Breaststroke or Butterfly?

I've been swimming regularly for fun and fitness for 2-3 years now and have become reasonably proficient in freestyle and backstroke. I swim on my own--no coach or masters group. I'd like to learn the other two strokes. Which of the two should I tackle next? My guess is that breastsroke would be easier to learn and easier on the shoulders but I'm game for either one. Thanks for any suggestions.