I Agree with one comment made, that you can never have enough Arty.
But surely what you choose ( depending on availabilty and points) depends primarily, if you are attacking or defending ?
If I am attacking and I can confidently target the enemy,being in a resonably static position, I want to shell them until they are quite useless as a defending force, either dead and on the way to hell or badly shell shocked they want to go home to mum.
The larger the Arty the better, I like to see craters the size of swimming pools and chucking shells the size of Volkswagons will do it for me every time. Arty.