Personally I'm partial to the Korky. I have put Korky's in all 4 of my toilets because I got tired of the noisy, leaky, squeaky Mansfield ballcock/float fill valves.
I have noticed that there is a bit of a religion about Fluidmasters though for some reason. they always looked dodgy to me, and way too cheap for me to consider something reliable. Am I missing something about them?