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.
2008 E92 M3: Sparkling Graphite | Bamboo Beige | 6MT | CF Roof | HRE | Catless Eisenmann Race | BPM Sport | Swift | Arkym | Mode Carbon | M.Perf