Yes Costco has nice price for watches. But you are looking at two completely different model. "You get what you pay for" in watches is true. Nighthawk and Skyhawk model are super accurate compare to the one you bought. Pilot, military, and navy people are the likely people who want Skyhawk model that's why it's more expensive.
Also Sapphire and Titanium is just the exterior of the watch, the internal cost a lot more.
Umm... Don't all the citizen Eco-drives have the same quartz movement? I don't see how one can be more accurate than the other. Plus they're both ATs so it's not like you have to worry about the accuracy sice they sync up every night...