My manual says to use 98 octane unleaded if not performance gets worse
Over here, 98 octane costs approx. $1.65 a liter while 95 is 20% cheaper.

Now, in my previous car (Opel Vectra GTS V6), I used 95 octane while my brother's 330i was on 98 and we drove to Greece (4500km both ways) in both cars. Gotta say we were going quite fast on those highways and my bros car's fuel consumption was 10% lower than mine and once a filled up 98octane somewhere in the alps, my fuel conasuption dropped by 10% and could feel better response in the alps.

My conclusion: Higher octane gives better performance and lower fuel consumption.