Observed also, that 90% of landings are accomplished using F30.
The difference in fuel consumption can almost be neglected. I read something in the range of ~15kg/approach. On the other hand, additional noise is created by drag and 8-10% more power with F40...
As mentioned before, in low visibility ops (esp. when flying manually CAT I down to minimum) it becomes much more easier with F40, also better landing performance therefore strongly recommended on CONT. or short and wet RWYs by giving you additional safety margin...
I personally think that both very heavy A/C and very light ones can be better controlled using F40...
F30 can be more desirable in gusty condititions with high Target-Speeds to avoid reaching placard speed for F40(158kts in the classics) and therefore to activate the reliefvalve retracting flaps to 30°...