In general, Monroe and Flat Rock are both pretty safe areas. Side-of-the-freeway hotels are pretty much the same anywhere you go in my opinion. Flat Rock is a little bit closer to Pte Mouille than Monroe, but it's not significant, so go ahead and make your hotel choice based on price. Like Denny B. said, there's a lot of construction on I-75, so if that kind of thing drives you nuts like it does me, you may want to find a route "off freeway."