One reason I prefer Iron Hill over Carousel - at Iron Hill I can blame the trees for my bad shots.
Carousel can be tougher to follow (even with a map) and it can get overgrown with thorny roughs (and pedestrians, and deer ticks). I'm not a fan of the last few Carousel holes, they just seem like repetitive open throws.
Carousel is much tougher to find coming off of I-95, but it's truly beautiful, especially considering its immediate surroundings.
Maybe a controversial statement but I think Iron Hill is the most challenging disc golf I've played. Nevin is also very long and woodsy, with extremely punishing rough, but the mixed open/tight, left/right, long/medium length holes help with your attention span. Some courses wear you down mentally because you can't find the tee pads or baskets or your discs in the rough. Iron Hill wears you down with long straight wooded disc golf. A very well designed and maintained course that challenges you mentally and physically on just about every hole.