The V90 has more cargo room and a bigger tailgate than the V70 - that could be a factor in your decision. Friends have a V70 and I like the way it drives - feels a bit more agile than my S90. That said, the 90 is a great highway cruiser; the straight six is very smooth, quiet, and has adequate power. The V70 will get slightly better fuel economy. My S90 can squeak past 30MPG (US) on the highway with conservative driving. My friends' non-turbo V70 averaged over 30 on a recent 3000+mi trip from CA to TX and back.

Overall maintenance and operating costs should be pretty close between the 70 and 90. Both have complex electronics, so plenty to potentially go wrong! Both engines will have long, trouble-free lives, given proper maintenance. I would be wary of either (or any) car if it comes with no maintenance records, as drivetrain repairs on these are very expensive.

The 240's are getting a bit old now, but if in good condition, can be very cheap to operate, practically bulletproof. They are rather low-powered, and not a very quiet car, but those who like them, like them a lot.
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Bob (son's 81-244GL B21F/M46