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.