In general, I definitely agree with doing your "top choice" second. Mind you, I think it was easier for me because my top choice wasn't my home institution. So, first I did a month learning from people I know and like, and then I felt very prepared heading out into the big bad world.
Key: I'd make sure that your away rotation choice has a good rep for teaching med students, particularly with regard to students from other institutions.
Also, if it turns out that you do your home rotation first, I'm sure it will be fine. For any rotation the key to success is interest and effort; since EM is your field of choice and your school is your institution of choice, I'm sure you'll show tons of both.
Good luck!
