MIVEC (Mitsubishi Innovative Valve-timing-and-lift Electronic Control) On the Evo it only controls valve timing (not lift)
Both the Evo 8 MR and the Evo 9 MR have six speeds

Here is a good list of most/all the differences.

IX has....
MIVEC
Better turbo(10.5 hotside vs 9.8)
Better, more addaptive ECU
Bigger fuel pump then VIII
revised coolent passages in the head which allow more aggresive tuning

blackout headlights and tail lights
revised enkies on the RS & GSR - 5oz lighter then VIII enkies
Hollow carbon fiber wing (VIII is solid)

Seats are now suede and alactra- black & gray (VIIIs are black & blue)
carbon fiber look dash piece
Steering wheel painted black (VIII was silver)

A IX normally dynos 10-20 HP&TQ more then an VIII. The MIVEC and new turbo on the IX make daily driving better due to power being available at a lower RPM.
The main reason a IX is better then an VIII is its outstanding output after mods.
A IX with just a turbo back exhaust and a flash will produce power similar to a VIII with turboback exhaust, cams and flash (and the VIII usually needs a Fuel pump too).