I've had both.
I've used the hemisphere ex's (with flakjacket, not armadillo) for about 3500 miles before I switched over to the thinner nimbus armadillo's for about 500 miles so far.
Moral of the story is, the armadillo's SLIGHTLY tougher only but the weight and rolling resistance penalties are huge. The nimbus armadillo @ 90psi rolls like a flakjacket hemisphere at 70 psi. Plus the hemisphere was a lot plushier a ride. Up to about 20mph, I didn't notice a difference in speed (mind you I mash a lot and there's a lot of stop and go traffic where I bike). Plus they handled the rough pavement a lot better. My tires were true as all hell for all 3500 miles. Two weeks after I switched over to the nimbus @ 90psi and tried to ride it like the hemispheres, my tires started going a little out of true.
I agree with supcom, while you're more likely to get f**ked in a front tire slide, I experience rear tire slides a lot more OFTEN.