While that's being settled I have a question for the OP. What did you intend the artillery to do? Take out bunkers or Battlemechs?
20 points of concentrated point damage from an Arrow 4 is good for Battlemechs but if its rubbling hard points or dealing with troop concentrations then gun artillery would be optimal. The Copperhead thumper only does a quarter the damage of a homing A4 but it you just need AOE, ammo is key.
I agree. My assumption for fire mission priority expectation, in order, would probably be
1 - Guided fire on target - the ability for any of the infantry suits to call in homing artillery (and LRM) rounds was my foremost assumption, being a solid replacement for long range firepower on the BA suits. This would also minimize collateral damage in still populated urban areas compared to spotted or plotted HE fire. The A4 has the clear advantage on this hitting 4 times as hard on direct hit.
2 - Anti Air umbrella - flak and ADA fire significantly threatens air assets. Both artillery pieces have capability here, I'd give the A4 the edge due to the ADA and threshold ability but the efficiency of Thumper flak would cover the role sufficiently.
3 - Special munitions deployment - especially smoke to cut long fire lanes allowing short range BA to cross/close. Thumper has the edge here. Thunder munitions inside an urban area is almost certainly a war crime but sometimes you've got to do it anyway.
4 - Counter Battery Fire - eliminate opposition artillery - advantage Thumper for range and ammo efficiency
5 - Spotted fire - HE rounds to flatten enemy strong holds. I'd expect to use this a lot less as civilian casualties could grow unacceptably quickly but still an option that may see usage. slight advantage Thumper for efficiency but speed of demolition goes to A4.
6 - Plotted fire - HE rounds to deny areas - Occassionally useful if, for example, reinforcements need to approach via a specific road. Big advantage Thumper for efficient ammo usage but with only 1 or 2 artillery pieces this would be a very limited option.
Overall for the UTC I'm still leaning towards the Arrow for the precision power of the homing round over the copperhead. I have however developed a new appreciation for the Thumper for more barrage fire.