I agree - as long as you have a 1080p display with a digital HDMI interface to the player, all 1080p Blu-Ray disk players have identical image quality - bits are bits. For smooth motion, as long as your display supports 72/96/120Hz refresh, you would also want 1080p24 playback capability which gives smooth motion and a very film-like image.
For conventional DVD upscaling, the relative quality of the video processer chip inside the player makes considerable difference. The three best video processor chips in order of declining quality are the Silicon Optix Realta HQV, the Silicon Optix Reon HQV, and the Faroudja DCDi. Very likely you will be more than satisfied with any player containing one of these when playing conventional DVD.
Audio quality is also important. I would buy a player that matches your speaker system (5.1/7.1) and your A/V receiver capabilities (SPDIF/TOSLINK or Bitstream over PCM).