I agree with Romeo, brigade artillery is best in the early days.
When you start getting Spencers and the like, you're better off equipping your close combat units with these and use the brigade artillery slot for an extra medic or something. This will also give you back some movement points which are important for shock units that need to penetrate deep into enemy lines or for daring flanking manouvres, to hit where it hurts the most.
One thing I never really got to work, though, was horse artillery.
The movement penalty really hurts the cavalry, I think, and I've never really seen that impressive results in fire combat.
Even a 'double' horse artillery performs pretty mediocre, is my experience.
Much, much better here, to give your cavalry a hard hitting carbine and leave them their full mobility.