Buy the one which has been better cared for. 15,000 km wear and tear is mere background noise compared to the damage which can be done by running a dirty or damaged air filter or failing to change the oil a lot. Find out whether the doohickey has been done on the 2004 or the doohickey spring on the 2008. Look for any signs of negligence in basic maintenance (chain, sprockets, fork oil). If everything else is equal, buy on the basis of upgrades, a.k.a. farkles: protectors, skid plates, luggage racks, maintenance free batteries, upgraded headlights, horns, fuses and switches can easily run a couple of thousand dollars.
If the economy there is half as bad as it is here, negotiate hard.
Hope that helps.
PS: Sounds expensive, though. A seven year old bike with reasonable mileage on it really costs the same as a brand new one here in the USA?