+1 on not having a fold-down plate.
Other differences:
The 591 has the USB port on the back, which is a preference thing (easier to do a permanent installation, or you can simply run the wire to the pocket under the radio and hide your MP3 player/USB flash drive there)
But you would already know that if you had read my reply in your other thread on the same subject
http://www.mazdas247.com/forum/showthread.php?t=123665394
Also:
- On the 491, the only knob does both knobs' job (you turn it for volume, press on it to access audio function, pull it towards left or right to skip tracks, up or down to skip folders). It can be a b**** to control while you're driving (you'll likely change track if you try to press on it to adjust audio options). On the 591, the controller at the right control the file/folder skipping. Less chance of changing track when you just want to change volume.
The 491 is not that bad, just need to get used to it
I still prefer having the USB port and the CD slot on the front
Also, on the 591 you can turn the internal amplifier off if you plan to use only outboard amps.