I was in the same boat, coming out of Madison. I was assigned MEM and got MSP as of Oct. 1st. In the meantime I am based out of MEM for Aug., DTW for Sep. and then move to MSP. So, if I were you I wouldn't worry about it too much. I don't know how they are assigning bases now though, I thought they were sending many new hires directly to DTW but I could be wrong.

But, if you get a choice right away, go DTW.