Is light or medium vehicle better for bombardment weapons?
Yes.
The comparison is between:
LVH+MB: 52 tons * 2 armor = 2.08 cost
VEH+2xMB: 98 tons * 2 armor = 3.92 cost, but 2x weapons so 49 tons/1.96 cost per weapon
On paper, this makes the VEH option seem a bit more efficient. However, each time a VEH is destroyed by a shot you will lose twice as many weapons, so the VEH-mounted artillery brings a bit more firepower but gives a more fragile formation. Of course VEH is a bit harder to kill due to having 4 HP versus 3 HP for the LVH, but I don't think that will make much of a difference in practice unless you have superior tech compared to the enemy.
While VEH can have 4 armor instead of 2, it is worth considering that doubling the armor also doubles the cost of the unit, so you are not going to be able to build as many weapons for the same build cost, although the weapons you do build will have ~4x as much survivability against counter-battery fire. As build cost is usually a major constraint for ground forces I would probably avoid armoring my artillery.
Personally I usually prefer the LVH variant as I prefer the more resilient formation that results, but either one is about equally good.
Note that if you care about optimization, it is difficult to beat STA+MB as these can have only 1 armor (thus are much cheaper to build) and can benefit much more from fortification than any vehicle type. Artillery does not benefit from the evasion stat of vehicle base types since that stat is only effective when a formation is in front line attack stance. In comparison, LVH+MB is really only a roleplay choice if you want to have mechanized formations. of course VEH+MB and heavier vehicle types still have some utility especially if you use the stronger armor to maximize survivability.