Chryslers, baby. They sound amazing in attack from far away. I'd place a bunch of them on the outskirts of the city, pointing in at it. I imagine that the ambience in the middle of the city would be breathtaking. If the city were too large, I'd stick some smaller ones in the middle; probably dual-tone ACA Screamers or SD-10s. Cyclones or P-50s would be awesome as well, but they'd probably be too loud for the occasion.
Don't know a whole lot about siren systems though; the only sirens in my area are at firehouses (although there is a T-128 at a dam).