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The Alcor and EC1 both use the Avago ADNS-3090, which is a very good sensor. Zowie uses a custom Kingsis made lens with the EC1, however, in order to achieve their low liftoff distance, and this lens also lowers the maximum tracking speed a little, to around 3.5m/s (compared to 4.5ish with the reference design lens used in the Alcor) depending on DPI and polling rate settings. The Alcor reportedly has a rather high liftoff distance, as many mice using the 3090 with the stock lens do.
The FK1 (which is a slightly bigger version of the original FK with a 24-step scroll wheel instead of the original's 16 step wheel) uses the newer Avago ADNS-3310, which is able to achieve Zowie's famed liftoff distance without a custom lens. It is able to achieve a far higher maximum tracking speed than its 3090 based predecessor, in excess of 5.5m/s. It also reportedly has lower jitter than many 3090 based mice, and a more constant report rate. If I were to buy a mouse today, it would be one that uses the 3310, and more than likely Zowie's implementation, as their implementation of the 3310 has proven itself to perform brilliantly, and their build quality is always solid (with the exception of an early batch of the FK14, which had problems with the rubberized coating).
The Kinzu V2 (non-pro version) uses the Pixart 3305DK sensor. It reportedly has many issues such as a low max tracking speed (almost as low as the WMO or Intellimice), making it unsuitable for low sensitivity players, high amounts of jitter, and significant input lag. There are also numerous complaints across various forums about the build quality of the mouse. I cannot recommend it.