As far as the miner to use, a large population will tell you to use cgminer, but I prefer phoenix because it gives me about 1-2% more mhash than cgminer (stupid variable intensities. even setting a high intensity is still variable, which is why its slower).
Not debating what you find in hashrate difference, but correction: high intensities are not variable, reporting looks like it is because of the asynchronous nature of the threads reporting back their hashes done. Only the longterm average is accurate and only after enough time has passed since it's an all time average of true hashes done which will include any dips across longpoll etc. There is no variable intensity in anything but dynamic mode in cgminer.
Sorry if I came off attacking/crabby, must've not been well rested when I posted it. I probably related it to DiabloMiner, which, when I used it, only allowed a variable amount of nonces per run and could only change it through a framerate argument, and thought if the hashrate was shaky, like DM, it must have a variable amount of nonces per run. But the async nature would make sense too, thanks for clearing that up.