87 octane on a V70 T5 with AC on all the time in sunny Phoenix, AZ. I have yet to hear any pinging problems. The computer should detect the fuel and retard the timing slightly so you end up with less efficiency. You should get slightly better fuel economy if you use premium fuels so I really think it's a toss up on whichever type of fuel you decide to use.
For some reason it took about 2 full tanks of premium before my fuel economy went up (31 to 33mpg).