Mileage is worse. BUT the biggest thing is how the car was taken care of. You could have a 1 year old car with under 10k that was beaten like a Mexican mule.
In terms of the E9x specifically, if they are well taken care of, everything should be ok.
Current: 2008 BMW M3 Sparkling Graphite/Bamboo Beige/6MT. 1999 BMW M3 Techno Violet/Dove/5MT. Next: 2017 BMW M3 CP