We have 3-month blocks and I did psych first. It was nice to start in friendly territory, and I was able to feel more comfortable in my first few weeks as I got the hang of things in the American system. But then when I went to medicine, I was the last experienced intern on the floor, since they'd all done 4 months of medicine (and they were already coming from a higher baseline, since they actually like medicine and our IM program is highly competitive). So I was in a constant daze of self-perceived relative incompetence for the first 3 weeks or so. That was compounded by the fact that my senior resident was well-known for his enjoyment of the sport of pooping on interns.
If I'd gone to IM first, my relative inexperience would have been much less prominent, since all of the interns would have been as fresh as I was.