replacing a chain - which coating type or make (nickel vs silver vs TI Nitride vs diamond like)?
In a different order than asked:


Weight: The differences in chain weight are well under your body's weight change during the day (or even a ride). Wanna save weight? Don't eat that bag of chips. It's not going to make a difference in performance.
Platings/Coatings: They're supposed to increase wear resistance and run a bit more smoothly. Not a huge effect, especially if you lubricate your chain properly. More expensive might be a bit shinier or work a bit better. Mostly aesthetic though. 
Price: Going with one of the cheapest compatible options doesn't hurt -- any difference in longevity and durability is far less than the price (and dirt will make the best chain probably die before the worst chain in a respectable manufacturer's line). I generally buy SRAM or KMC though, cause I don't like Shimano's rivet system. Obviously if you run Campagnolo, most of their stuff is not friendly with other manufacturers. 


I generally buy the second/third from the bottom chain in a product line -- you get chrome hardened-ness and sometimes chrome plating with it, so you get a bit less rust in winter.