Chauvinists...
DOES rocket artillery count? It's cheaper and easier to build than shell artillery - its only drawbacks are that it's slightly less accurate and it takes somewhat longer to reload.
Also, it can quickly and thoroughly saturate an area with explosives. Which is very important when you're trying to wipe out a beachhead or break out from one.
A rocket launcher is cheaper than a gun since it does not have to deal with recoil or the explosion stresses of firing a shell. Rockets can also be easily placed on simple truck mountings. However artillery can also be mounted that way these days.
http://en.wikipedia.org/wiki/CAESAR_self-propelled_howitzer
That transfers the cost to the individual rounds. A rocket (like a recoilless gun) requires much more propellant than an equivalent shell or mortar bomb. It also requires a rocket engine, and that is not too cheap - there is significant precision engineering involved especially on the nozzle/venturi bit. You can probably buy 4 or 5 122mm HE shells and cartridges for the same cost as one 122mm GRAD HE rocket.
The gun concentrates the cost in one big lump of engineering (the cannon and carriage) that is
not thrown at the enemy every time you fire a round. The rocket throws expensive bits of precision engineering down range with each and every round (compared to the plain artillery shell which is relatively speaking, just a bit of turned metal - and is much the same as the pointy end of your rocket). Thus the gun or mortar wins out in the long run cost wise over rockets. Tube artillery is also more accurate.
The multiple rockets advantage is delivering a shotgun-like salvo of rounds simultaneously on the target. 40 of them for a GRAD launcher for example.
http://en.wikipedia.org/wiki/BM-21_Grad
Those 40 (expensive) rounds then have to be reloaded before the thing does it again, which takes some time. Over time, the gun or mortar will be putting as many rounds or more down-range than a MRL, and not in "burps". The 40 round burp is sometimes tactically rather nice to have though - MRL are wonderful systems for breaking up an attack (when the attackers are out in the open and perhaps bunched up). MRL are therefore a useful adjunct to tube artillery but in no way are they a replacement.
The rocket is also useful for delivery of chemicals, or cluster weaponry. The rockets can have thinner walls, so hold more chemical or bomblets than an equivalent thicker-walled artillery shell. The 40 round "burp" of rounds is also the way to go for chemical delivery since you want a rapid concentration of agent.
MRL such as the classic GRAD mentioned above
are good for counter-battery fire on enemy artillery that is not too precisely located. Just fire a battalion (18-24 launchers) salvo at the estimated position. This is one application where the wider dispersal of rocket munitions helps, rather than hinders.
Some of the wandering rounds are likely to bother the enemy battery!
Different rules apply once you get to guided rockets (such as the present unitary warhead GPS corrected ones fired from MLRS - GMLRS -
http://www.lockheedmartin.co.uk/us/products/GuidedUnitaryMLRSRocket.html). Those have replaced the 175mm and 8inch howitzer systems in modern armies. The rounds are not cheap, though since you have stepped up from an unguided artillery rocket to a guided missile of sorts.
For your hypothetical application, the round is the most important bit. There is a saying in the artillery community that "the projectile is the weapon".
You want a decent supply of relatively cheap ammunition for your gun. You would like multiple suppliers so that your supply is reasonably assured. So that means most likely, NATO standard 105mm or 155mm. Several nations make that. If you can get hold of the British L118 howitzer then that is good - it also has a replacement tube that fires old American M1 105mm ammo (L119).
http://en.wikipedia.org/wiki/L118_light_gun It is towed and can be slung under a helicopter.
Mortar wise, 120mm is a decent choice for light artillery as well. Several NATO nations and others make 120mm mortar ammo. So do the former Warsaw pact nations and China. The Finnish Tampella 120mm is the classic example, you wont go wrong with that. The FRG bought a lot of those.