a while back on Batlabs someone posted some pics of those Chinese knockoffs (believe the antenna in question was a UHF Jedi 403-520 wideband) and took one apart and a real Motorola OEM version and put them side by side.
the Chinese copy was poorly constructed, and the radiating element was weakly soldered to the base on only one side, and had a crack, thus causing the radio to perform poorly and eventually (if used to TX alot) would have caused an expensive to repair PA failure.
the real Motorola antennas are the only way to go, or at least a reputable aftermarket supplier such as Radial/Larsen, Centurion, or Maxrad. Cheap crap from China is like going to Wal-Mart for a pair of shoes. You get what you pay for.