Basically, the MP4 is skinnier and brighter, but uses special lithium batteries, and doesn't have the red LED.
The MP8 is a little thicker (same weight apparently), and has the nifty red LED, but you lose out on some of the brightness, and the runtime is slightly less. The advantage is that it uses AAA batteries.
I posted a link to cheap CR123A Lithium batteries on eBay, but there are more savings to be had, like using the MP8 model with rechargable batteries, preferably bought from Woot of course
Great flashlights either way!