Avalanche currently hits for ~2/3 of the damage of an ele force proc.
It has ~5PPM vs ele force's 10 RPPM. Its really hard to compare PPM w/ RPPM since PPM directly depends on the number of attacks where RPPM is event independent. Perhaps avalanche could be better for boomkin/affliction if you have 2 dots on 5+ people, simply because ALL of those can proc avalanche, while any dot tick resets the proc rate on ele force.
Both are currently slated for nerfs though in 8.1.5 according to the PTR. Based on the current PTR, I imagine 9SP will be a popular option. Ele force still procs a ton and is going from "fucking OP" to just "good". Coastal surge is also getting nerfed, but an additional HoT is pretty nifty for a number of healers.