a 10 year old diesel with 120,000 miles is low in my eyes.
id personally go with the 120k diesel as it has history to back it up... BUT depending on price.
many 'low' mileage diesels are driven around town but demand a higher price, a 'high' mileage diesel is most likely to have been driven on the motorways and will be cheaper due to the mileage.
when we were looking as civic diesels all the ones which were in my parents price range were just under 100k or had gone well over.
mileage means nothing these days as long as its got history to back it all up, cars can easily clock over and beyond 100,000 miles with no problems... my uncles civic diesel for example, its an 2006-06 reg, he has had it from new, its been serviced 4 times at honda and the rest by myself, its had 1 clutch in 195,000 miles and it was replaced about 3 weeks back.
it also depends on what you mean by patch history aswell. has the 'low mileage' one been serviced yearly but late or has it missed services?