difference in materials used on the exterior.
old version is glossy plastic on the sides with a semi rubberized matte finish on the top. the new version is harder rough plastic with rubber side grips added.
difference in side buttons.
the old versions buttons were slightly bigger and much easier to press. the new version has stiffer buttons.
the new sensor is higher dpi (though honestly this is not a huge deal)
then things such as glow color (blue vs green), the fact that the new scrollwheel has a black band and other non-important things.
its all about preferences. i just like the older design better myself. the new one isnt bad (i tested it) but it just doesnt suit me.