Tiger926, everytime I have checked Southwest's airfare from Detroit, it is almost always higher than Northwests prices, and it is not nonstop. I guess Southwest just does not have great airfare from Detroit.
I have usually always flown Northwest from either DTW or Flint, but this year Spirit had some awesome rates. So, I ended up booking with them.
Here are the prices I found for your dates. I'd say if you wanted to buy now, then I 'd pick Northwest. Especially if you are using Magical Express. I have never paid over $200 on Northwest to Orlando. If you are willing to risk it, it seems like Northwest usually lowers their airfare the closer you get to departure. Last year I went 7/10-7/20 and I bought our tickets in late April, they were $170. However, I noticed that in June the prices went down to $116.
Here are the prices I found for your dates.
Spirit $210.90
Southwest $234
Northwest $230.90
Good Luck!