Congrats! Many Red and Buff roses to you!!
I guess this is a greedy way of looking at things but I would look into the future and say..."Would I advance in pay and position quicker and more substantionally if I do the internship? or is it just a resume stuffer?"
You may love the quick promotion but I would way out what you could get if you waited and did the internship....I know nothing on what you do at your job, but either way it sounds as if you would move up quickly. So I dont think you have a prob either way!
Good luck!!